In the case of Uber/Lyft the driver has another function besides being the driver: they are on the hook for the capital invested in the vehicle or their ability to lease one. That's a lot of money per vehicle and utilization is the responsibility (and risk) of the driver.
This is true, but note that for a self-driving vehicle, utilization is only bounded by demand and maintenance. An Uber driver can get tired or busy with other things, but a self-driving car can go 24 hours a day minus re-fueling/charging.
