What competitive market? There is no spread, no bidding. Buyers and sellers cannot negotiate. Uber decides prices based on some proprietary, secret algorithm.
"We are going to pay you $X, if you don't like it you can leave" is called a salary, not a marketplace.
Not only that, but uber and lyft copy each other's prices. When one of them lowers the pay to drivers, the other one follows soon. Usually, the pay to drivers is lowered, but the cost to riders doesn't go down as much, and uber/lyft has been taking much more from drivers, especially the 'surge' surcharge. Much less of that goes to the driver than before.
"We are going to pay you $X, if you don't like it you can leave" is called a salary, not a marketplace.