> All of this means Uber, and other ride-sharing services, must lean heavily on pricey incentive payments—cash for completing a certain number of rides a week, say—to bring driver earnings above what typically amounts to around minimum wage

This seems kind of obvious to me. Uber's main drawcard for drivers is that there's almost no barrier to entry. Most jobs earning more than minimum wage have some significant barrier to entry (required qualifications, experience, skills, knowing the right people etc.) and so if Uber pays more than minimum wage, anyone in a minimum wage job will switch to driving for Uber. This lowers individual earnings for drivers until other low-barrier-to-entry minimum-wage jobs become competitive again and drivers start switching back to those jobs.