In the areas where Uber is prominent, it's hard for low-skilled workers to find any employment at all. Consequently, some of them turn to Uber. This means that no matter how bad the wage, Uber has a ready supply of drivers.
It's extremely important for Uber to make Uber a cost-effective alternative to driving or public transit. The drivers are less important to Uber -- both in the short and in the long run (automated cars), the drivers are the expendable part of the equation.
(However, do note that under certain circumstances, Uber subsidizes the drivers: sometimes they are paid more than the passenger is charged.)