Because AT&T have limited bandwidth (the physical constraints of the universe prevent any other option), and they offer "unlimited" service based upon the avera

Because AT&T have limited bandwidth (the physical constraints of the universe prevent any other option), and they offer "unlimited" service based upon the average usage pattern of phones (+ a few standard deviations) being within their networks capabilities. The usage pattern of computers is significantly different (p2p services, background downloading, etc.) so in order to be able to offer unlimited service for that usage pattern they need to increase their own limited bandwidth significantly. If you want "true" unlimited bandwidth (using your full download speed 24/7) you would have to pay a order of magnitude more for the service.