Some people actually do not want to be at home (I once had a taxi driver who drove us from Manhattan to Brooklyn, then waited for us to go home, hours later, all the while texting his family that he had to keep working).
Also, leaving the car turned off and waiting in a lucrative parking lot saves gas, wear and tear on the car, and drastically decreases the opportunities to get into an accident. All in all, it is not a bad plan. Taxi drivers have done this for years (which is, ironically, probably one of reasons Uber got started in the first place -- in SF it was almost impossible to get a cab, because so many cabbies just wanted to get airport fares).
That's what happens at airports. The drivers are there anyway. While they wait, they coordinate to increase pricing just before an airplane drops off a bunch of passengers.
Also, leaving the car turned off and waiting in a lucrative parking lot saves gas, wear and tear on the car, and drastically decreases the opportunities to get into an accident. All in all, it is not a bad plan. Taxi drivers have done this for years (which is, ironically, probably one of reasons Uber got started in the first place -- in SF it was almost impossible to get a cab, because so many cabbies just wanted to get airport fares).