They'll never go that route because Google wants full control on your device. But yeah fundamentally, there's no technical reason why Android couldn't be a normal Linux distribution.
We are back to square one and what I said originally yes, they don't want to do it because they want full control on your device.
There's no technical reason why Android can't just be another Linux distribution. There's no technical reason why you could not boot into an OS from a usb key on your phone like a normal computer. There's no technical reason why the play services have more rights than the actual owner of the phone. It's all about who controls your device.
Android follows a "look, don't touch" model and will continue to be this way as long as possible on Google's side.
