Microsoft wouldn't have nearly the install base without the Series S, and developers can either target that platform or not, just like the Switch, because people bought it for its own strengths. If they want to scale their games up to a spec such that it runs on PlayStation but not Xbox, they're welcome to, but they lose access to a large pool of customers, like those who can stomach paying $300 for a console but not $500. There are plenty of other next gen open world games that work on Xbox.
Also, your analysis on how it should perform isn't really based in reality. We can go to interviews where the Swen Vincke calls out the way their game does split-screen specifically. And besides, at this point, Xbox engineers are involved, and BG3 will run on Xbox, though likely just next year.