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.
Yeah, okay. Until you hear stories like this.....
https://www.techdirt.com/articles/20131012/02124724852/decad...
And yet even telecoms have invested huge swaths of capital to get us to 3G then LTE.