That is how it is woth other games as well, it is not unique to BG3. e.g. the Pathfinder Kingmaker, when it launched, some players complained that the highest difficulty is too easy, some players were really struggling on the lowest. Eventually the devs caught on and added a lot of options to the custom setting. There will be always people who, even excluding exploits, are more skilled at using character builds, even single classes.
Customization imo works better than simply using difficulty presents. For now in BG3 unfortunately, even in custom mode, you can only choose a preset for enemies, but maybe they will add more options.
I'd add that BG3 does have some issues, notably lack of poper party control where it adds to the difficulty. Because effectively you either position your party before combat, or you run into combat while your party holds no proper formation, like you are herding a group of chickens.
