The problem is that they basically habe a nonaggression pact. You'll find either AT&T or T-Mobile in any given area; not both. This means that, if you don't like their prices or their service...well, what are you gonna do? Build your own telco?

This lack of competition leads to a sort of mini-monopoly, meaning that they basically have no reason to not jack up prices.

That is, until Google comes along and kicks them out, as another comment points out.