I can understand point #1

- 99% of people put a case on their phone

- the more thickness you have for the camera (sensor+lens), the better you can make the optical design. (bigger sensor, more range of focus, etc)

- the camera - especially wide angle lens - must be even with surface of case, not below it (otherwise the case occludes the edges of the photo)

Pixel phones also have a camera bump, but it's a band symmetrically placed on the back on the phone. Which is why it's stable.