My theory goes that they must be super busy creating this amazing story with cinematic, romances, dialogues; and still have little clue and how exactly the gameplay and rules will evolve.
Larian has grown a lot though, and narrative/cinematic departments aren't working on combat systems. Some of the Larian's oddities that persist in BG3 existed for three games now, so I don't hope that 1.0 be much tighter designed then what we have now. Narrative hasn't been a strength of previous Larian games and while much more effort is put into that side of the experience, and as of now I am not convinced it will land in "fine" category, especially that they seem to riff on D:OS2 a lot.
I'm fully expecting a Core Rules difficulty setting where none of the poor homebrew exists.
Wouldn't that be nice, but I don't expect to see anything of that sort. All my hopes rest in modding support, and modders with far more programming and design competence then myself.