Nothing prevents the drivers to long-term lease a robocar like a personal vehicle and send it to work for Uber during the time when they don't need it.

Currently an Uber driver can drive at any given moment only one car for Uber. With robocars, a driver can invest in 2, 3 or more robocars and send them to work for Uber. Similar to how people buy multiple properties to rent out on AirBnB.