DRM and Content contracts. They can't just throw 4k into an unsecured environment because they need to make a best effort to enforce that their 4k content isn't pirated.
Windows/Apple have special APIs which allow video decrypting to be done on the GPU. And the GPU will not allow you to access the decrypted video surface.
This is why you can't screenshot Netflix on Windows/Apple - the region where the video is will be black.
It's not. If you can see it, you can copy it. It just makes life harder for a small number of legitimate users on Linux or rooted Android phones because their license/contract says they need to use DRM.
And Netflix exclusives regularly make it to pirate sites in 4K afaik?