It takes close to half a billion dollars just to test it. Stuff has to work right the first time, and every time after that, for 40+ years. They (and by they I mean we) test the human factors of the displays the controllers work at, the communications links between the ground and the air, the changing of routes for thunderstorms or wind changes, the interoperation of communications and navigation systems built across several decades, the algorithms for deconflicting merging aircraft and each type of aircraft's ability to follow the deconflicting instructions... stuff's complicated, yo.