I'm reasonably certain it's a combination bandwidth and tech issue. The Vision Pro is effectively using AirPlay to mirror the whole screen. If you used AirPlay

I'm reasonably certain it's a combination bandwidth and tech issue. The Vision Pro is effectively using AirPlay to mirror the whole screen. If you used AirPlay to mirror each window as a whole screen, you'd run out of bandwidth pretty quickly. The windowing system in MacOS, Quartz Compositor, also isn't built to stream window information. Right now it has a big built in assumption that any windows its displaying are on a screen it also controls. It was probably too big a lift across teams to also re-write the graphics stack for MacOS for the launch of the Vision Pro. Hopefully they get it working in the future, but neither of these problems are easy to solve.