You probably spent a lot on your iPhone, and you don’t want something like your car and house keys scratching it up when you cram all of them into the same pocket.
Sure, you could slap a case on there. Or you could keep your keys and phone in separate pockets. That’s what I started doing, but that was mainly because I had a hole in the secondary pocket, and the phone was the only thing that wouldn’t fall through it when I walked.