I've wondered this myself and thought about it a bit.
My conclusions were:
1.a. People like to decorate their phones as they see fit (proper armor would make cases that much larger)
1.b. The ability to decorate a phone using after market cases is important to the phone manufacturer since more people are likely to buy a phone if they can dress it up as they would like.
2. Aftermarket products have varying levels of protection, the consumer can choose what suits them (a woman with her phone in her purse all of the time has different needs than myself, I carry my phone in a pants pocket).
So they focus on thin phones of various rectangular sizes.