Unfortunately, it's all about appearance. A back cover that pops off _appears_ more fragile to most people. Similar thing with water resistance. They could ma

Unfortunately, it's all about appearance. A back cover that pops off _appears_ more fragile to most people. Similar thing with water resistance. They could make a resealable back cover. However, you now have support trying to determine if someone's phone has water damage due to manufacturing issues or because they didn't put the cover back on. This leads back to the phone appearing more fragile (or Apple seeming petty). You also have the scenario where the phone is dropped hard enough for the cover to come off and then land in water.