Some drivers will be actively delivering their passengers to their destination but be available to queue up the next pickup.

Other drivers without passengers will either be re-positioning to an area they consider a hotter spot for activity, will be doing other daily tasks/chores/getting food while still searching for passengers, or, as another redditor wrote, be parked somewhere waiting for a passenger.

The cars are not exactly where they truly are (for the safety of the driver), and usually the app shows fewer cars than are actually available.

You're also not just going to get the nearest car -- Uber matches drivers and passengers for what makes most sense economically in terms of pathing and pooling.