Because you are buying a device without the feature of installing a custom OS. If you want a feature, buy a device offering it.

The whole point of this discussion is the trend towards societal scale denial of that feature by both market forces and governmental forces.

There is no such a thing as "just buy a different device" when this "different device" is actively discriminated against to the point it's a paper weight. I wouldn't be surprised if remote attestation becomes necessary to even get an internet connection in the future.