They aren't profitable because they're burning money on crap outside of matchmaking riders with drivers.
They're pissing all that money away on loosely related r&d and their hopes/dreams of an autonomous future. They now also need to somehow continue justifying their mind boggling 72 billion dollar valuation.
I expect in the coming years Uber will die and be replaced by a service that works exactly the same way but for a far smaller revenue take and without any of the lofty ambitions.