That's their business model today. CallmeSam was referring, I think, to their predicted future model based on a fleet of self-driving cars. In that scenario they do indeed buy and own the cars and make a killing by offering them as a service in competition with their own huge fleet of drivers. It will, I would think, be a very awkward transition for them, and creates the possibility of their drivers defecting en masse to a rival like Lyft before they can roll out enough self-driving cars. But if they have enough market dominance they might pull it off.
However, the fact that Tesla, Google and possibly Apple may decide to operate rival fleets (and may even decline to sell uber the best self-driving cars on grounds that they make more margin operating the ride-sharing service and all the network effects that brings them) will create an incredible competitive landscape. It's shaping up to be one of the great land-grabs of the 21st century. At some point, if one of the manufacturers seems to be losing the race to the others, it has an easy master-stroke of going into partnership with uber.