Xbox series S have 8GB for game, so while BG3 might consume around 4-5GB on PC, console with unified memory couldn't afford this. All the other assets(model/animation/audio clips/massive amount of icons) needs to be loaded as well. With split screen, you can have one person tries to go into conversation (that streaming in high res texture/face models, etc) while the other one stay and still render the world with all the things their camera can move around with.