Last time I checked Uber still loses money. It's because they spend a vast amount on marketing both in the form of ads and subsidizing rides to undercut their competition. ie. In certain markets they are paying drivers more than the fare charged to customers would indicate. (Also, uber spends a lot on frivolities like driverless car research and branding redesigns, food delivery, etc.)
This is because Uber believes and has convinced investors it can become a worldwide monopoly by killing off competitors, network effects, etc. But the evidence so far is not supporting that case. Lots of competitors still, lots of markets that are "lost" for Uber.
So the real problem is that in the long run they can easily be profitable, but probably not at a level that justifies their valuation, since what they provide is a commodity. But luckily for them, this won't matter, because they'll IPO and be dumped onto the 401ks of witless Americans long before anyone figures this out.