If I want a camera running a smartphone OS, I use a phone. A camera should be by definition running on dedicated camera firmware, and nothing else.
Says who? There's no intrinsic reason a camera couldn't run with an Android OS. In fact, there's a lot of good reasons why you would want that - simpler development platform, reusing existing drivers, etc...
Basically an add-on for your phone that adds a serious interchangeable lens sensor.
There is: battery life and startup time.
DLSRs have no problem being being on standby for weeks if not months with minimal battery drain and then springing to life within a second at the press of a button. Android phones do no even come remotely close to that level of efficiency.
My M1 air is in standby for weeks at a time on a single charge. There's no reason you can't do similar with a phone. Maybe not months, but definitely for extended periods of time.
What camera needs: fast "boot", stability, reliability, ability to run offline for decades.
And no, I don't want all software being developed the way a social.media app for a phone is.