A city bus carrying 20 people has 5% the drivers-per-person that an individual vehicle has. The driverless system only would save 5% the human effort that it would for a solo vehicle, so it needs to be 20x as good before it's worthwhile. Put another way, the bus with 20 riders is already equivalent to automating 19 cars.

Roughly the same idea with freight - a single truck driver serves the needs of many people.