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.