I’ve noticed over the years that FaceTime is much more likely than other video chat software to drop the video connection and move to audio only in case the connection is unstable whereas most others will hitch and lag for 30 seconds before looking into it, so maybe they got around it by only shipping the video in one or two resolutions?

How many participants can you have in a FaceTime group call?

I have also noticed that FaceTime drops the video much more often that other software.

Basically it's doable, but if you can prevent people complaining about the fans taking off and the CPU usage... why would you risk it?

(No idea how widespread encoder/decoder support is compared to vanilla h264 though)

[1] https://en.wikipedia.org/wiki/Scalable_Video_Coding

Zoom automatically switches between quality levels based on your connection speed, who's talking and the size of the viewport. 720p would look fairly rough when fullscreened on most non-mobile displays, but it's orders of magnitude more than necessary when viewed as a thumbnail on a mobile device. Making multi-user video work in a mostly seamless fashion is a surprisingly hard problem.

Using a single stream would substantially degrade the experience, which may be a worthwhile tradeoff for high-security environments but certainly wouldn't be a worthwhile tradeoff for most users.

I may be spoiled with a good real 50/10 Mbit connection but for me in 2020 720p is the bare minimum. Expecially when screen sharing.