[marklubi]

This would be like cutting off their nose to spite their face.

Apple profits from phone sales. The more distribution channels, the more phones they can sell, and more organizations spending the money to advertise iPhones.

If Apple bought a cell carrier, they would be in direct competition with the other carriers. Worst-case, they would likely get dropped from the other's lineups. The less worse case, they wouldn't get advertising/promotion from their competition.