To be fair, these drivers would be out of jobs without Uber. They are not qualified enough to find better paying jobs. Being underpaid is still better than not being paid at all. However, there's a limit to how little Uber can pay its drivers before they start looking for better paying jobs elsewhere. Uber needs to balance this with their need to be profitable in order to keep existing. And they're not profitable already in spite of these low wages. If anything, it's Uber who needs to worry about its existence. Drivers will find jobs elsewhere.
I think Uber started as a high margin business, which is always a great place to be but they quickly turned into a (very) low/negative margin business because of competition which requires exponential growth and constant injection of money to pay for the growth. The amount of pressure on the ceo must be insane, glad I'm not in his shoes (doesn't justify him being a-hole to his driver though obviously).