So how come you can't (legally) watch Blue-Rays using a Linux computer, or when viewing it on an ancient CRT using an HDMI-to-analog converter?

A lot of effort has gone into making physical media work only with pre-approved devices.

It fails on all of them if it's not available to purchase, and none of them are of relevance if I want it right now vs having to wait 1-7 days to get hold of that physical copy and there's an easier alternative where I can have it right now.

Physical media has arbitrary device limitations.