Regardless of my personal feelings on masks, this is a poor argument. If I'm using the Uber app, and the app says drivers will wear masks, then I expect drivers to wear masks. If what you're saying about brokering services is accurate, it still is Uber's business, because if they are promising their riders something and the limo driver fails to deliver, then it's in their interest to break ties with the limo driver (or to not make promises to riders anymore)
I'd be willing to bet that in order to be on Ubers platform, they require the drivers agree to wear masks.