edit Supporting comment apparently wasn't appreciated. No delete button.
It’s voluntary. If it becomes unprofitable, drivers leave. If they lose money, they won’t do it. They give a share to Uber because Uber is their lead generation. Many businesses give a share of their money to Google because Google brings them business. An Uber driver can market their own business and keep all the money if they want just like a business could go out and market without using AdWords. If you are unhappy…
It also takes sufficient life-situation stability, good health, and so on. Shaming people for lacking "ambition" is neither universally appropriate nor universally effective.
EDIT: I didn't originally upvote the parent comment, but I now have. I think there's a very interesting debate to be had when we consider individuals as economic actors. Disadvantaging individual participants in the "gig economy" too aggressively (e.g. by prosecuting Uber drivers for collusion) forces us towards solutions like unionization by dividing us into privileged capital and downtrodden labor. I'd like to talk more about we might empower individual actors (e.g. by improving the social "safety net" and making the job market more fluid).