Because pay for the drivers is awful when you consider the actual cost of maintaining a car that runs for hundreds of miles every day. So the only ones that survive are those that do not do proper maintenance and those that game the system (e.g. by reporting fake customer no-shows).

This is exactly it - Uber tries to have "fancy cars" as compared to cabs, but there's a reason cabs are made out of washable materials; the first time someone vomits in the back of the Uber it's never really coming back out (there's a "vomit" button in the driver app apparently, it's that common).