I agree it's not the same from the user point of view. However from Uber's point of view - cost of 24 hours to rental company vs cost of 24 hours of taxi+drive

I agree it's not the same from the user point of view. However from Uber's point of view - cost of 24 hours to rental company vs cost of 24 hours of taxi+driver would probably be a good approximation to get an idea of the order of saving which might make owing self driving cars feasible for Uber vs their current business model.