The problem with automated air transport is that the failure mode is going down through gravity and probably not being able to land on a suitable surface. I doubt we have automated pilots developed so far that any failure (and those are a lot!!) can be handled, and even then you might want to manually control.

The failure mode of automated driving is less severe. You're already on a road, gravity holds you down and unless the brakes don't work (although in an electric car or bus, when not accelerating you're braking) the likelihood of accidents is smaller.