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.