I was reading here and there topics about multiplayer, especially based on some "server-oriented" services (Steam, Galaxy, GfWL etc). I was wondering though - why so few developers nowadays are deciding for multiplayer mode(s) less "dependable" on services, which can be shut down in the future? Like LAN (with things like Hamachi or Tunngle it's possible to play via Internet, if I'm correct) or TCP/IP (so connection between two (or more) PCs etc?

Is it harder to develop? Or developers just don't care about the future, because "now it's working"?

I admit that I'm partially interested in this topic because it's also connected with DRM-free games, those "less-dependable" methods of multiplayer seem to be more "durable" than those based on some services.

It would take time to implement and they just don't want to bother since they can also use servers. No one cares about the future, obviously. Then there are also companies like Blizzard who want to control everything and force you to use their servers and whatnot (Starcraft 2 ist the best example for that).

I think it has more to do with longer term plans of constantly "monetising" the player with pay2win, micro-transactions, loot-boxes, etc. The next big thing for multi-player will be "skewed match-making". This is where they'll deliberately mis-match two players then throw pay2win adverts at the weaker player to "encourage" him to buy upgrade to match the stronger one. EA have already patented it:-
https://www.pcgamesn.com/ea-matchmaking-microtransactions-eomm-engagement-patent

Likewise, if some content is server driver (Diablo 3 style), then it's also prone to even single-player online games having the mechanics "played" with (server-side AI deliberately changing mechanics mid-game to "encourage" pay2win). Eg, in an online ARPG, you could pick up 3 out of 4 pieces of a matching set, then the AI will pick up on that and deliberately prevent the 4th from spawning for free but offer adverts to buy it via micro-transaction. Or deliberately make a boss more grindy than normal vs your equipment but whilst offering "time-saver" (pay2degrind) levelups or ability to buy better equipment that's strong specifically vs that boss just at the right "peak frustration" moment:-
https://www.techpowerup.com/240655/leaked-ai-powered-game-revenue-model-paper-foretells-a-dystopian-nightmare

Obviously this stuff can only be done server-side for online / micro-transaction laden games. Old-fashioned custom 3rd party servers or offline LAN play will be immune to screwing gamers over with it and hence offline play is now "undesirable" for the 'modern' AAA industry...

I think the effort to write a reliable LAN stack compared to an off-the-shelf solution which you plug in and mostly works, is a lot higher. So people naturally take the direction of the least effort.

This, and while LAN is generally pretty easy to set up and work with I remember TCP/IP connections being an absolute pain back in the day... might not be so bad now with broadband but my memories of playing online back when that was the norm are of spending more time trying to set the game up and get everyone connected then actually playing...

And I was fairly tech savy even back then... imagine asking today's modern generation to mess around with settings like that when they're used to just hitting a button and having it done for them?

If a developer can just plug into a ready made system like Steam rather than going to the cost and trouble of developing their own then of course they will. It's what most people expect and given the choice what most people would use.

It would be nice to have more options for things like LAN, but I doubt that's in high demand these days either.

There seems to be some confusion here - LANs still run using TCP/IP so you have to program it anyway. And for multiplayer UDP/IP may be a better choice for most genres.
First, you need servers for matchmaking. Second, P2P connections have one huge problem - cheating. Sure, you can cheat with client-server but it's not as trivial. You can see Diablo post-mortem where David Brevik talks about it here.

But you don't need those servers if all you want is to play a game with a few friends.

We have had zeroconf/avahi/bonjour for over a decade now. Adressing on a local network isn't difficult. Trouble is traffic between networks, most end users are behind NATs, most users are not proficient in administering their NATs, and allowing that to be automated is a massive security risk.

Well, yes and no. The players still need to connect with each other. For 2 player games it's not much of a problem (though you still need to know the IP addresses), but with more players, one actually has to act as a local server to which the players connect.
Ultimately - it's a problem of how much control the developers are willing to give to the players.