Two main points:
- no one unified distro to keep things simple (thread OP)
VS
- people don’t care. Someone else needs to advocate, sell, migrate, and support (medium term) Linux (whichever distro they want) for the intermediate term (few months at least) - thread response).
I think a lot of the 97% desktop market share is like this, instead of the hands on 2-3%.
It’s open-source merely to comply with the GPL license of the kernel, but the fact is that an Android image built only from open source components will be extremely crippled or, depending on your point of view, basically useless. Such an image will not even boot on the majority of devices ; you’ll need those sweet proprietary driver blobs if you want your phone to do anything, and a bunch more closed source binaries in order to use Play services.
Linux has a similar issue though
Don’t know why this is downvoted. Even distros like Debian, that are 100% FOSS otherwise, will (often) load some proprietary firmware because otherwise it would barely be functional on any modern computer.