The reason is (the potential for) self driving cars. Once/if there is mature self driving tech, they can eliminate the cost of having to pay human drivers. When you consider Uber in that light, as long as they don't run out of money before self driving maturity, then their high valuation makes sense.
This may seem obvious to some but this fact has been entirely absent from every conversation I have heard about the valuation of uber's IPO ( looking at you Marketplace's "Make Me Smart" ;). It seems like it is taboo to point out that Uber needs to eventually eliminate all their human drivers from the equation.