If Uber were cheaper than the depreciation and running costs of a car, what would be left for the driver (and Uber)?

a big part of the whole "hack" of Uber in the first place is that people are using their personal vehicles. So the depreciation and many of the running costs are sunk costs already. Once you paid those already it becomes a super good deal to make money from the "free" asset you already own.

My private car provides less than one commute per day, on average.

An Uber car can provide several.

While your car in sitting in the parking lot, the uber driver is utilizing their car throughout the day.

The depreciation would be amortized to cover more than one person. I only travel once or twice per week, it cost me less to use an Uber than to own a car.