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