Then people would create browser plugins or greasemonkey scripts to always optimise for things the viewer cares about (time to start, likelihood of switching providers mid-stream, likelihood of getting full resolution for the longest subset of the video, ...) and disregard the prioritisation set by the provider (which might care about costs, which depend on contracted minimums, overage tariffs etc.).

Then providers would need to combat this by dropping the most expensive CDNs, causing a race to the bottom in which everyone loses: users have worse streaming experience, providers lose customers, good CDNs make less money, margins for bad CDNs are squeezed.