When there are more people who prefer convenient hours than shifts available, some will only have the option to take inconvenient ones. As hourly employees, Uber can't use dynamic pay to manage supply and demand imbalances.

I dealt this as a teenager working at McDonald's. Most of us didn't want closing shift, and occasionally at 25 cent/hour incentive would be added. But more often than not, the demand for any hours was sufficient. Labor supply simply exceeded demand.

Still think Uber and likes should be regulated away until we have true self driving cars. That may take a decade or more.