Because it's cheap and easy!
There are certain markets where there's just no feasible way to compete with the entrenched players. If some company wanted to compete in GSM, they'd have to buy spectrum (oh, AT&T and various other providers own it all? And AT&T specifically owns all the world-standard GSM spectrum? And they don't want to sell? Too bad). On top of that, they'd be fighting against a company that could just drop their prices until the new rival runs out of cash, and then raise them back up. That's basically what monopolies do, and why we make monopolistic practices illegal.
You CANNOT go to other providers to GSM except in name only. They only "compete" as long as AT&T allows them to, and their calls will always have second-class status on AT&T's networks. That means if it gets busy, your phone is the first one to be cut off.
I'm guessing it's your ignorance coupled with your misplaced sense of superiority that makes you come across so stupid. Something like the Dunning-Kruger effect.