This depends on the agreement that the media distributor has with the media owner; there's no technical reason behind the 720p limitation. Widevine is offered

This depends on the agreement that the media distributor has with the media owner; there's no technical reason behind the 720p limitation. Widevine is offered with different levels, depending on the length of the secure path. On mainline Linux, there is no secure path to the screen AFAIK, so you can always read the unencrypted video from a recompiled graphics driver, and as such it only is supported in Widevine's lowest level.