Larian designs their games to be jampacked full with enemy encounters. And due to the lack of long resting restrictions in the game (and Larian's design tendencies leftover from DOS1&2), they kind of have to assume that players will come into each fight with most, if not all, of their resources to ensure non-trivial combats. Thus, each fight has to be on the Hard to Deadly level. When we add in xp for exploration/quests/other non-combat solutions, using "proper" MM XP for each of these fights would result in leveling up way too fast.
Using 5e XP guidelines, it's roughly 3-4 Hard/Deadly fights to reach each of levels 2 and 3, and then on average ~8 Hard/Deadly Fights to reach higher levels. In Act 1 of BG3 (not counting Grymforge), there's over 30 combat encounters the average player will find if they visit all the areas. Using Proper XP, this would take characters to level 6, maybe even 7, before reaching Grymforge. Again, not counting exploration/quest XP.
