Part of the issue, is that the entirety of combat is centered around Larian's homebrew rules moreso than 5E, so adding a 5th party member isn't just about adding another person who can attack/cast spells etc. It's another person who can push, create surfaces, carry/throw barrels, abuse broken sneak.
That is going to completely take the existing imbalance, and ratchet it up more than a few notches.
Given that Larian's encounters are already balanced for PCs that can do those things, it's debatable that BG3 encounters/experience would need to be adjusted significantly more than PnP 5e encounters would. After all, the enemies can push, create surfaces, often free disengage, and backstab too. And no encounters should be balanced assuming barrel usage or abusing sneak, so that's not a problem.
But either way, it's vastly easier to adjust the single "Experience Gained" equation than adjust every single encounter, considering many properties such as the # of enemies/enemy abilities/enemy hp/etc.