If one could find FCC cellular frequency licenses to build another A T & T or Verizon cell system, at todays costs, the Licenses cost alone would be over $150 Billion. The hardware, labor and materials to build out the system would take another $250 Billion. Barrowing that amount of money, to build a competing system would leave you with an annual debt service of $20 Billion, equivalent to the net profit of T or VZ.
Sprint is the only other cell company that could ever compete, providing they can continue to barrow the money to chase the big guys without going under.