Pretty much all party-based cRPGs have some system of party management where you can access everyone's models and inventory simultaneously, and rearrange your party composition: original BG games, NwN2, DA, PoE, Pathfinder. So BG3 not having anything along those lines is indeed yet another major way in which Larian has done a poor job with this game so far.
All those games let you change companions in and out all the time no matter what act the game is at. In bg 3 that wont be the case. So they think they don't need it just for one act.
