Because it doesn't meet the requirements for L2. I think L2 implementations are required to block software screen recording, for example, and there isn't really any practical way to enforce that on an open platform. Windows/Android/iOS have special support for compositing protected content so if you try to read the framebuffer back the content just shows up as a black rectangle.

L1/L2 requires a third party who could be liable to sign that the drivers are unmodified to the hardware.

On a general purpose Linux installation who would do that?

(And who in the Linux using community wouldn't take any efforts by someone to try as an afront, bluntly).

Also there aren’t really enough of us watching videos on Linux for it to be a worthwhile market for them to address, I think.

DRM only really works if you're not root on your own machine, and with Linux you're always root on your own machine. Quite frankly I think DRM (the normalization of rootkits) is dangerous.