It's really funny to me to see people trying to come up with all kinds of convoluted spin to try and justify Larian's choice about locking us into only three companions and no more after Act 1. There's really nothing to explain or justify. It is absolutely a terrible game design choice, something that fundamentally takes away from the roleplaying element of the game and thus degrades the game as an RPG. My only hope here, just as with too damn many other aspects of this game, is that it can be removed/fixed with modding.