My iPhone is being "used" on it's screen. If my screen scratches or breaks, that's a completely different thing than the back of it getting scratched. The back of the phone is cosmetic just like the exterior of your car. Because, just like a car, I'm always "using" the screen and the back of it is merely what other people see while I'm using the phone.
If the back of your phone is what other people see when you use it, you must be holding it very daintily by the edges.
The back of the phone is always handled and susceptible to wear.