No, actually, I'd tell you all cars rust to some degree, it's expected.
Nevertheless, if your phone scratched it was because you chose not to apply a screen protector and performed some operation that scratched it that involved abrading the screen with something harder than 5.5 on the Mohs hardness scale (the hardness of the iPhone front glass); there's no other way it can occur.
Note the most common mineral in sand, which is the most likely source of a hard material in your pocket or unknowingly on your finger, is quarts, Mohs 7.
Again, why?
Because glass can be scratch-resistant and brittle or more scratch prone and less brittle.
You can apply a $30 screen protector to prevent scratches; you can't do anything other than use a case to prevent the screen from shattering if dropped, and most prefer to spend $30 on a protector than pay hundreds of dollars to replace a shattered screen.