In most cities, there is more than enough demand for drivers to work full time (~40 hours/wk), but they value the flexibility over maximizing pay.
There are plenty of folks out there that are unable to work 8 contiguous hours. Perhaps they have children, school, or even a part-time job.
If a college student has free time, they can earn cash as a Uber driver, but when they have a final exam, they don't have to work a second. Traditional part-time jobs, like being a barista aren't that flexible, so the alternative may be having no income during college.