(Assuming non-EE here)

When you start BG2, you will have everything in your inventory for a brief moment (probably just a few frames), but it will disappear immediately as the very first cutscene takes everything away.

A few specific items will show up later if you had them, but most of your items will be gone for good, so make sure to use those stat tomes before exporting your character.

(Note that, if you open your inventory fast enough (before the first cutscene starts), you can drop your items on the ground, and then retrieve them after the initial cutscene, but that is not what the developers intended, I believe.)

Only two items plus the Golden Pantaloons can import over, but they are, with everything else in your inventory, removed. Imoen, when she rescues you, even tells you that 'your stuff' has 'probably been sold'. The two items that can be imported come from this list of possible items. See the BG2 table. The best items to import are Helm of Balduran and Chainmail +3, though you only need the Chainmail +3 in your inventory (assuming you have none of the others), since the helm is the default prize. Note that the 3 things you import are all found in the first tutorial dungeon, so you don't need to go hunting for them later. Only the helm will serve you the entire game, anyway.

Thank you

Such pity, because in BG you collect and collect items, but then when you want to continue you cant.

I hope they will correct this, or at least make it available to buy all your stuff back.

You could use the trick I mentioned in my previous post:

Immediately on starting the game, before the first cutscene starts, access your inventory and drop all your items. Then, the cutscene will not take them away, and then you can pick them up.

You have to be quick, however, and I believe this does not work in the Enhanced Edition, but it is an option.

It's kind of a pain that you don't get to carry your items over, but it makes sense if you think about it. Most prisons in the west don't carry firearms (there's a few that have pepper guns, tasers, or other non-lethal weapons). This is because if a prisoner manages to grab a gun, the result would be catastrophic. It's the same kind of logic. Keep in mind the "inmates" of the dungeon aren't common street thugs. Like the residents of Arkham Asylum they have supernatural abilities, and a few might have martial training.

If Irenicus kept your gear lying around, there's always the chance you'll escape. And if you escape, you'll be well equipped to fight the hazards scattered throughout the dungeon - as well as Irenicus himself. The items you get in BG1 don't scale well with BG2 IMO. It won't be long until you find better gear.

One major problem with this explanation:

It works fine from a story perspective.

It does not explain the gameplay side of things. In particular, why is taking away the player's items good for gameplay?

Gameplay-wise, it's easy to understand why they did this, it was simply to limit the power discrepancy at the start of the game between people who imported a character and people who started a new one. In a RPG, it's always hard to find the right difficulty to both challenge the most powerful characters and yet keep the game doable by weaker ones.

Making you start fresh in terms of gear at least allows the devs to reduce that power difference, and it also enables them to control your gear progression more easily.

It won't be corrected, since this game is ages old. Though there may exist a mod to change it, if you really want to.

But either way, even though you can't buy your items back, the vast majority of BG1 items can still be found in BG2, either in stores or in the hands of other enemies. (Though some items work differently in BG2 than they do in BG1, for example the Ring of Wizardry.)

Anyway, BG2 items are generally much, much stronger than BG1 items. By the time you are halfway through chapter 2 it's doubtful you'll really miss your old stuff.

The same goes to BG Siege of Dragonspear, there the companions of the main character whom you leveled for a long time have disappeared (with all of the loot ).

I understand the BG2 problem, but this is overkill. Their you are in a prison, but here you are a hero of Baldur's Gate and all of you companions have left you without saying anything leaving you with the weakest party members.

I completely agree. But the complains should be addressed to Beamdog who just did not bother to justified any of its gameplay choices.

I don't really see this being likely flavor reasoning.

Irenicus is (rightfully!) unconcerned about what you might be able to do. He's been throwing around 9th spells before you ever met him, and likely before you were level 1. Iirc, pre-BG2 he captured your party in its entirety before any of you could so much as get a good look at him (when killing tends to be easier); later, you know what you are up against, and he still does it again. Exit the intro dungeon, and he stands off multiple cowled wizards. He's probably about as concerned about your party being a threat as I would feel threatened by a couple of fire ants; they might sting a little, but an out and out fight would be no contest.

I hadn't really considered it before, but my guess is your stuff simply became bribes, payments, and armament for Irenicus' allies and minions. Does "Hey, go do this task for me and I'll give you a shiny trinket" sound familiar? ;)