The business model where there are a ton of part time drivers ready to jump to action to pick up a hail for a few dollars of much needed extra cash is a good one. Riders are able to get a ride anywhere and anytime that way.
Their business model is not meant to support a fleet of full time drivers, as there is not sufficient fares out there to keep full time drivers busy. They need cars everywhere, which means part timers on call for the next hail that is close by.