The problem is that in this case, Uber doesn't own the car network. It's trivially easy for a driver to shift between networks, or even work for multiple networks simultaneously. Uber not only has to talk a rider into giving them the fare, they also have to talk the driver into taking that fare. There's just nothing special about Uber for either party unless Uber can be the one that performs that matchmaking service at the lowest possible transaction cost. Welcome to Uber's low-margin world.