Dustin Curtis explains:
… it turns out that when you hold the iPhone in your left hand and articulate your thumb, you can reach almost exactly to the other side of the screen. This means it’s easy to touch any area of the screen while holding the phone in one hand, with your thumb.
This is an example of one of those design decisions that you don’t usually notice until you see someone doing it wrong. It’s one of the things that makes Apple products Apple products.