So... I think with BG3 being a 'DND'-type game, i'm pretty sure you don't get EXP for doing talking in DND, you gain experience from combat. At least that's how i understand it, from the games of DND i have played..
That may be why it's the way it is.
The Dungeon Master's Guide actually includes basic instructions for 'Milestones' and 'Levelling without XP,' (P.260-1)
Personally I always DM with milestone XP because my group has a mix of roleplayers and power gamers. The power gamers tend to want to maximise their XP and feel like they're missing out if the roleplayers circumvent fights with creative roleplaying solutions. So milestone quest XP keeps everyone happy.