Because Steve Jobs pwned Randall Stephenson and made him his bi.....
Seriously though, Apple actually cares about aesthetics and I'm sure that was near the top of the agreement. Same reason they don't put Intel stickers on their Macbook lines. People buy Apple because 1. It works and 2. It is made by Apple and looks amazing. The
is a fashion statement. People don't want crappy corporate logos on their beautiful, work of art iPhone.
I'm an art student and that was one of the biggest gripes with my older cell phones. Plus they are usually stamped on cheaply or stickers that peel off leaving weird marks. It was nasty and I hated it! Blah!!