With the added benefit of it being one number to call, rather than any of several cab companies (or hundreds, in New York).
The idea of "ride sharing" is somewhat dubious, but the enormous win of Uber and Lyft is to be able to go to one place (well, one of two places) and just get a ride. If cab companies had just banded together for a single dispatch app, or even a single 800 number, they might have shortcut the process before Uber arrived on the scene.
