Because half the population doesn't have front pockets that fit phones, and sometimes your purse is slightly out of reach.

I keep my phone in my front pocket 90% of the time. However, there's been a number of times where I've been distracted, and have slipped it in my back pocket just before sitting on it, such as finishing a conversation just before getting into the car or working on something that may get my front wet, like gardening. It's a nice adrenaline rush feeling it under me all of a sudden, but, thankfully, I've never inflicted damage like that.

Because you can't sit down at all then. The note 3 (which I own and love) is too big.

Keys and change go in front pockets and tend to scratch phone... Phones fit in back pockets and I've never damaged one that way in many generations of iPhones (and also Android phones).