I confess I know nothing of Uber's running costs but in my layman's understanding I think GP point is still valid. Driver buys the fuel and services the car. How does having more rides cost Uber more?
In the past they did this to achieve market dominance over taxis and lyft because they were drowning in billions of dollars of VC money, and the whole point of getting all that money is to become a market leader as soon as possible, even with loss-leading strategies.