For the average non-technical user, a "pure Linux" phone with no sand boxing would be a security nightmare. Just look at jailbroken iOS devices as an example. Maybe sophisticated users know how to take extra precautions in exchange for more freedom, but the average user will happily use banking apps with no idea what the other processes on the phone are doing.

For this reason, Android is a good middle-ground. It's well-sandboxed by default and if you need more power you can root it. Nothing is stopping anyone from releasing "free software" open source android apps published outside of the play store. As is, they would get more adoption than any app on the Ubuntu/Firefox/etc store.

As long as Android exists and is easily rootable, and preferably as long as the iOS jailbreak community keeps up with major versions, I'm happy with the state of "linux" on mobile.