They don't agree voulnatrily on that low payoiut per mile because Uber and indivudual drivers have very different bargaining power. A free market can only exist between well-informed rational actors that can both walk away from the deal. In the face of huge power disparities you need limits on the power of the stronger player for a long term market to exist. This is exactly what minimum wage is. So don't trode out the tried old propaganda of cut-throat capitalists such as "they took the deal".