Because if the case gets damaged you can easily replace it.

People would still put a case on a bulky phone to protect resale or trade in value.

A super thin phone doesn't require a super bulky case, it requires just as much case as a person would normally use, resulting in a smaller overall profile.

I'd probably still go pro because I care more about the camera than the size.

They're sort of doing that with Ceramic Shield, and the new bumper case for the Air.

I think the reality is that your phone will always encounter something that can damage it (unless it's made out of diamond), and a case lets you easily replace the damaged part.

Just subjectively, I remember having a super scratched iPod and it just felt kind of ratty every time you looked at it. Meanwhile, a phone in a leather case gets kind of a patina that improves with age. It is kind of sad though, I got a really pretty blue iPhone and you wouldn't even know it because it's completely covered by a case.

Honestly, it sounds really compelling to me. I do a lot of stuff outside - search and rescue, climbing, etc, and I need a rugged case for that, but having a thin, light phone when I'm at home or doing something with less chance to damage the phone is pretty nice, so thin phone + case is the best of both worlds.