Employees cannot be paid a fixed rate per deliverable. They must be paid an hourly rate, or be paid a fixed salary per time interval, regardless of the number of hours worked.

But the employer can mandate when and where an employee works. So, for example, they can require that you are on the road a certain number of high demand (surge) hours. They can also require that you respond to a call to pickup a specific passenger within a certain time period, etc.

So the result will be their algorithm will ensure the hourly rate they are paying a driver is covered in the long term by the rides they are serving, or they could simply fire that driver.

In a complex system, you could specify time slots which you would promise to be available, their system would determine a subset of those slots where they wanted to hire that driver which would work out to be profitable enough for them. Their system could suggest an alternate set of (partially overlapping) times to make a counter-offer of when they could hire you. If the driver didn't show up for work on the peak hours they really needed you enough times, the driver would be terminated.