the games difficulty scales not only with your actual characters but it expects you to learn and understand its systems, from 3->4 you don't actually get that much stronger but the fights scale a lot more
but i do think it's kinda ridiculous a lot of people complain about it and then don't have any new skills learned the past 20 hours and aren't using the insane abilites that the entire plot of act 2 is getting
5e/bg3 is the exact opposite of that though. you can't get better than full plate- unless you get something with godly effects, so realistically your act 1 equipment and weapons once you get a +1, can carry you the entire game probably
and since your levels give you everything you need, you can very much go through the whole game without touching a single merchant and just beelining quests
divinity is a crpg strategy game, the part of about evolving your strategy and learning from your mistakes and the difficulty stemming not only from an increase in the games difficulty, but also from expecting you to understand more about it