Everything is already compressed. Uncompressed 1080p is 3Gbit/s, which is already well beyond what's actually achievable with WiFi. Allowing an unbounded number of window surfaces to be streamed invites people to use it in a way that current technology simply cannot provide the kind of slick experience that Apple needs from this product.
Seeing as you have the eye tracking you could probably greatly reduce the FPS/bitrate of windows that are out of gaze. This kind of foveated rendering seems to already be used in their ecosystem so I assume its sufficiently slick for Apple. Apps will sleep when not being gazed and such.
This allows you to restrict full bitrate to a single window with a maximum resolution and keep the user experience high and astonishment low.
