AD&D =/= 3 =/= 3.5 =/= 5 =/= Pathfinder
Comparing it is counter-productive.
I can level up to 150 in Fallout 4. Why can't I level up to 150 in Pillars of Eternity or Baldur's Gate 2? They claimed to be RPG games aren't they?
To me personally, and I think most reasonable people would agree:
Leveling up purpose should be only about to build your character. When your character is -built- there is no purpose to move all the way up.
Yes, it is possible to level up beyond level 12, but does the story/encounter design/level design supports the new mechanics offered by higher level spells?
It's a choice that have been made by Larian, the answer is no.
5e still goes up to level 20, BG3 does not.
Theres literally zero character building with just 12 levels, even with 20 theres hardly much you can do.
Most feats and such are now automatic, the whole reason why people dont like BG3 and 5e level caps is because theres far less possibility for character building.