Which is a cost, because then you have to call around trying to find someone who is willing to do it then, which is exactly the thing the app does for you.
In practice you have one or maybe two people you call, and then fall back to using the app anyway if that fails. So the person comes out ahead, in that they have a decent shot at a guaranteed ride, better service, and lower cost. The absolute worst case scenario is the standard app experience.
