(This post applies to both games in the series; until further notice, it is safe to assume it doesn't apply to BG3.)

Now, these games doesn't perfectly replicate the AD&D rules; some rules are not implented for good reasons, like the demihuman level limits (widely considered to be a poor rule, and would make most non-humans useless come ToB), and the fact that Wish doesn't match its tabletop counterpart (accurately implementing that spell in a CRPG is not feasible; the closest you could come would require some fancy AI). However, there are two rules where the implementation differs from (what I remember of) the tabletop game, and the game suffers from it IMO.

1. Mages don't get to choose new spells at level up. As a result, mages are at the mercy of the scrolls that the game provides. This, in particular, doesn't feel right when the game also features the 3e sorcerer, whose biggest disadvantage being a limited spell selection (containing *only* spells learned at level up); it just doesn't feel right that sorcerers can get certain spells before mages can. (For comparison, the SSI Gold Box gave magic-users 1 spell per level up; 1 made sense because of the limited spell list, but 2 is what (IIRC) the rules say.)

2. Death and dying. In the tabletop game, there's the death's door rule, where characters don't die immediately at 0 HP, but instead remain alive until -10 HP. This makes being a little unlucky (especially at low levels) less punishing; if the rest of the party can kill the enemies, the fallen character can be restored to health, and (from a CRPG perspective) a reload is not necessary. Only if the character goes down to -10 HP and the party has no side-effect free resurrection is a player going to need to reload (though the "drop everything on death" rule subverts this, unfortunately). On the other hand, in the Baldur's Gate series, characters die *immediately* on reaching 0 HP, which is especially nasty at low levels where just one unlucky roll means a dead character. Even worse, on Core Rules or harder, if a character is reduced to -10 HP, the character is gone *permanently*, which makes it pretty much certain that the player will reload; this is extremely player unfriendly design, and I also consider game design that encourages frequent reloading to be (in most cases) bad game design. For comparison, the SSI Gold Box games implemented the Death's Door rules (though it *is* worth noting that Raise Dead isn't side-effect free, though Resurrection is).

So, any reason why the developers chose to mis-implement these rules in a player hostile manner?

1) Mages in the P&P games need access to a library to learn new spells. They can also copy it from scrolls. A spell they don't know would have verbal, somatic, and material components that they are not aware of. So it would make sense that they need to get the information from somewhere else. Sorcerers learn new spells at level up because magic is just part of what they are. A sorcerer's ability to cast a fireball is like you or me moving our limbs. Magic also works like this for monsters; pit fiends and red dragons can throw fireballs as a natural ability. Sorcerers are closer to a monster than playable characters when it comes to lore.

2) This would be game breaking. The game is unbalanced enough already. Almost every monster is watered down. The end game for BG1 and SoA is resist fear, haste, and then zerg everything in your path. Boss characters are an exception to this, but that's usually how it goes. This works for a good portion of ToB as well. There are a ton of rules that won't plug in to the IE games well. Like the lethal damage rule.

I've added screenshots of the scanned player's handbook (AD&D second edition). It seems that you're confusing those with later editions, because:
1) It highily depends on the DM and the situation whether you get a new spells on level up. You definately need a source for that no matter what. In the game you can either find or buy the scrolls.

2) This rule is terrible and it was added in D&D 3rd edition and I don't like it in the pen&paper RPGs. There needs to be consequence to bad decisions/luck. You can almost say that "dying isn't fun gameplay", just like Larian said that "missing isn't fun gameplay". Where's the challenge then? If you're dying too much, maybe try lowering the difficulty? The low levels should be hard to survive, I even think the whole game should be like that. No matter how experienced you are a sword to your face will be deadly.

Neither of the rules you refer to was a fixed rule in AD&D 2nd Edition. The automatic extra wizard spell at level up (it was only one, not two) was merely a suggestion which a DM could use or not. Also it said that the DM could decide whether the spell would be chosen by the player, by the DM or at random, and if the player chose it then they didn't learn it automatically, they had to make a "learn spell" roll. And if they were a specialist then the new spell had to be of their specialist school. (Also of course in the pencil-and-paper game the specialist's extra memorised spell at each level had to be of their school - I can't think why this wasn't implemented in the game, I can't imagine it would have been all that difficult, though I'm not a programmer.)

Something else to note is that in P&P wizards would typically start with more than three spells known at first level. There were several specific suggestions for DMs to choose from, but all of them would result in more than three spells unless (in some methods) the player was very unlucky with the dice.

The "death's door" rule was an optional rule presented in the Dungeon Master's Guide. The standard rule was that 0HP meant death. Even with the optional rule, a character on 0HP or less would lose 1HP each round unless they received a cure spell or some sort of first aid, and even after being restored by anything less than the 6th-level heal spell they would be unable to fight or do anything much until after at least a day's rest.

As for demihuman level limits, as you say it's pretty clear why they weren't implemented in the game, but their absence does mean that if you want to play a class available to a demihuman race, and to stick with that class, then there is basically no reason to play a human because of the racial bonuses demihumans get.

It was present before, as an optional rule, and the SSI Gold Box (which are based on 1e) implemented the rule. This I disagree with, When the party is low level, it is likely that the player is new to the game, and therefore the game should be gentle in order to allow the player to safely learn how to play it without getting too frustrated.

Once the player has gotten used to the game and gained a few levels, *then* you can ramp up the difficulty, but not before.

(Note that this assumes that the player doesn't try to go somewhere that they aren't intended to go to that early in the game. Also, this assumes the player hasn't set the difficulty high, in which case the assumption that the player is inexperienced no longer makes sense.)

Even if the racial level limits were implemented, they would have almost no effect on BG1, so the problem you mention would still exist there; in ToB, the limit is crippling enough that it makes non-humans completely non-viable. (Humans in BG1 and BG2 are at least viable, as the racial modifiers and traits, while nice, aren't necessary for the character to function; being able to reach appropriate levels, however, is.) Just take a look at Pools of Darkness; you can create non-human characters, but non-human non-thieves are capped at levels below what humans start as, so a party of all humans is really the only way to play that game.

A better approach would be to do something like 3e did; instead of giving non-humans a crippling limitation that only comes into play at high levels, give humans a little bonus that comes into play right from the start. This way, there would be a reason to play as a human single-class character (other than racial restrictions on class selection), and non-humans would still remain viable.

(Note that there is a difference between "viable" and "optimal"; from the standpoint of a CRPG, it should be possible (and not overly difficult or tedious) to complete the game with any reasonably balanced party of viable characters; if some of the important roles (or worse, the entire party) are filled only with non-viable characters, then the game would become unreasonably difficult and/or tedious, and possibly impossible.

I would disagree about the game being too easy, particularly for newcomers. See this recently posted topic, for example:
https://www.gog.com/forum/baldurs_gate_series/baldurs_gate_is_impossible

Also, your "endgame" strategy is complex enough that I wouldn't say the game is "easy" at that point, particularly since one of the spells you mention, "resist fear", is not something I would generally think to use. The game is certainly *not* so easy that you can just auto-attack every fight and heal afterwords and expect to get through the game.

Also, this particular rule tends to have the most impact early in the game, when the player is still learning how the game works and needs all the help they can get. At higher levels, when damage amounts can reach the double digits (allowing a character to be taken from living to dead even with the death's door rule), and whe resurrection magic is readily available and death isn't usually so sudden, it's not needed so much.

As for the mass damage rule, the problem with the rule is that it doesn't scale at all. If I have a few million HP, why should I have to roll a saving throw just because an enemy dealt a measly 60 damage to me? (Of course, by the time the game gets to this point game balance has already broken down in other ways, but that's another story and the point still stands. AC and THAC0 can (in theory) be made to scale sensibly at high levels, whereas the mass damage rule can't be unless the constant 50 is replaced with something that scales.) Dying can be fun gameplay, but only if it's reasonably fast and easy to recover from it. (See games like Syoban Action/IWBTG and Super Meat Boy/VVVVVV/Celests for examples of games where death is extremely common, but can still be fun.) When death results in a game over or a situation that can't quickly be recovered from without a reload, then it becomes not fun.

Missing isn't fun for a different reason; it makes the game RNG-dependent in a rather frustrating way, where it feels you aren't making any progress, and where losses feel like it isn't your fault. (I would rather die to Syoban Action-style traps (which can be easily avoided if you know where they are) than to the RNG saying "no, you don't get to live", particularly late in a long battle.)

Incidentally, one house rule idea I've toyed with in my head is to adapt something like the Life Points used in much of the SaGa series. They would allow for more survival near death (you lose only 1 LP per hit that would reduce HP below 1), but not be so easy to restore (can't restore it with a basic healing spell the way you can HP). This would make players want to avoid losing LP, but death would be relatively unlikely unless the player does something risky like using the Diehard feet (under this rule, any time you'd lose HP while using the feat you lose 1 LP instead).

Both early editions of D&D and BG had a different target as a player than what we have now. Right now almost anyone can be a player, so the skill spread is extremely wide. 20 years ago most players were what some would consider as hardcore players now. BG is not for everyone, I don't think there's a game for everyone.

Therefore I don't see a point of making a game easier, because there will always be someone for whom the game will be confusing or too difficult. You would probably end up making the game easier and easier, until it's just basically handholding and not fun for those that want challenge and complex mechanics.

BG has a difficulty slider already, why not use it? Besides this game already has a following even after all those years, I don't see a point of making it easier to increase the fanbase. Those that enjoy that type of games and are willing to learn will do just fine.

There are way too many games right now and most probably need to select thier niche. I don't have problem with that, I prefer if the game is more unique and takes risks even if that means fewer players will play and enjoy it.

That’s exactly what BG1 does, with a tutorial area (Candlekeep), followed by an easy road with new companions to enroll in your team (Friendly Arm Inn, then Beregost / Nashkell).

During all the way from Candlekeep to Nashkel, difficulty is really progressive and concepts are not all thrown in your face at once:

1. In Candlekeep, you start with a single character by fighting enemies that can barely hurt you (assassins) or even not at all (rats, friends with staves). You can train with a team against enemies that can’t hurt you (illusions).

2. In the wilderness towards the Friendly Arm Inn, you fight against weak enemies that can only use hand-to-hand attacks, that are first encountered alone (gibberlings, wolves) then in groups (xvarts). Against singles enemies you play two characters (Imoen joined the team, you can use ranged combat), against the groups you play four characters (Xzar and Montaron joined the team, you can use combat magic).

3. In front of the Friendly Arm Inn, you fight against your first mage. If you lose this fight, you have an autosave very close to this encounter. Then your team grows to its full size (Khalid and Jaheira joined the team, you can use heal spells and buffs).

4. On the way to Beregost, you may encounter your first enemies that use ranged combat (bandits). By the time your run into them you have a full team including a tank (Khalid) and are stuffed with health potions.

5. In Beregost then in Nashkel you face your first encounters in interior, where good team positioning is crucial. They are all against single enemies.
Due to these encounters being triggered on area transitions, you have an autosave before each of them.

6. In Nashkel mines you learn about traps, and how to handle big groups of (weak) enemies.

7. When leaving Nashkel mines, you have to fight your first powerful group of adventurers. If you manage this fight, using all you learned until then, then you know all you need to complete the game ;)
Once again, there is an autosave prior to this fight.