Pretty simple really, we're all human and it only take a small mistake to scratch an iPhone. I'm not sure what is so difficult to understand.
A screen protector only protects from scratches, not drops on the corner of the phone that result in a rather large bag of fail. The screen is scratch proof, that is why apple banned the sale of screen protectors in their stores. The only screen protector I have seen that doesn't look terrible is the one that comes with the otter box. Those are pretty legit.