The Uber driver(s) are all self-employed businessmen, with Uber and Lyft being apps that help them get business, taking care of things like payments etc. As a businessman, you take a risk, especially if making big investments like a $100K car. The 3rd party, in this case Uber, has no obligations to the businessman; if the driver wants more money, then stop relying on Uber or go to a better-paying competitor.

That's business. That's the American dream. That's the risk you take when starting your own business. Uber couldn't exist if it hired their drivers, so for Uber too it's a conscious decision.