There's a 20 factor test, I don't know how current it is, but one of the stipulations inside of it is:
The extent to which services performed by the worker are a key aspect of the regular business of the company. If a worker provides services that are a key aspect of the company's regular business activity, it is more likely that the company will have the right to direct and control his or her activities. For example, if a law firm hires an attorney, it is likely that it will present the attorney's work as its own and would have the right to control or direct that work. This would indicate an employer-employee relationship.
Now in the case of Uber, driving someone to their destination to me seems like a major part of Uber's business.