Driving for Uber is as flexible job as it gets. I don't see how it's flexible for Uber - Uber can only offer a ride if a driver decides out of their own free will to accept a ride.
For many people (2+ Million), driving for Uber is worth the money. They decided that it's a better gig than their other options. If they decide that it's not worth it, then they stop. And because it's such a flexible job, they don't even need to give two weeks notice.
Even though you think that driving for Uber isn't worth the money, millions of people around the world do.