I'm going to go a little tin hat here. Maybe WotC told Larian custom char gen is not allowed. If there is a mod that allows you to change the rules to match 5e rules and have a cool character editor, it will compete with the 5e subscription service, which they've been pushing in the boardroom for quite a while now, using words like "D&D is undermonetized." Maybe a little too far on the side of conspiracy theory that Larian is not allowed to let people make custom-generated characters.

WotC would also oppose a DM mode for BG3 for the same reason. WotC were being complete dicks to other game content producers when they tried to yoink the license for 5e out from under them all, and the new contract says you can't use the license if you animate a magic missile on screen. (I haven't confirmed that; I just heard the "rules lawyer" say that on Youtube.) It would make sense they would gimp Larian if Larian tried to do some things that could be used outside of BG3 for a TTRPG, like a custom character or dungeon maps.