[c_o_n_v_e_x]

There were far fewer planes in the air before GPS. In order to squeeze more planes into the same volume of air (aka "airspace density" or "airspace efficiency") you require high performance navigation systems. GPS isn't just used for navigation (getting from point A to point B), it's also for helping maintain separation between planes.

The US's navaid infrastructure is old. Not all airports have the "luxury" of ILS.