Balance reasons. It's probably pretty fucking hard for them to create open-world acts where you can run into all sorts of trouble and shenanigans, and have them be challenging instead of just allowing you to steamroll them with your 8 party members. Or, imagine if the world was designed for a larger party. Enemy counts would have to be that much higher to compensate, and battles could take much longer.
They had to compromise somewhere. 4 was a good amount in DOS2. It works for BG3 just fine.
Same as every other party-centric RPG
When are we going to get a volcel companion who does not engage in pre-marital eye contact?