Because that's farthest away from your head, and minimizes radiation absorbed in your head (SAR); yay for regulations.

Besides, it's not like you can put it somewhere else inside the phone and not be guaranteed to block the signal -- again, the whole head thing. The design goals of a mobile phone are seriously contradictory -- good radio performance at 850-2100MHz (wavelengths 14-6 inches) in the smallest possible package, and "normal" conditions involve being held directly against a loud-mouthed waterbag which must be protected from excessive radio absorption. The design is always a compromise, and it's honestly pretty incredible that most mobiles work as well as they do, especially considering all the additional obstacles (camera, battery, etc.) further restricting the available antenna space in a modern smartphone.