@Gargron I'd argue there's a critical reason besides rent-seeking: security.
It's a genuine conflict between user rights and the need to protect the average person. Phones hold our banking apps, 2FA tokens, mics, cameras, and countless secrets.
When a sideloaded app steals data, the user doesn't say, "My sideloaded app failed." They say, "My Android/iPhone got hacked." The OS developer takes the blame.
Android's approach—allowing it, but behind a clear security warning—seems like a decent compromise in this difficult balancing act.