On top of that, the threat model of potential obstacles in the sky is far simpler. On an airliner you have a heading and speed for every tracked aircraft in you

On top of that, the threat model of potential obstacles in the sky is far simpler. On an airliner you have a heading and speed for every tracked aircraft in your vicinity and the buffer zones are far greater, even accounting for relative increase in speed. It is much more difficult to predict and quickly respond to edge case obstacles and erratic agent behavior on the road. Perhaps there would be an analogy here if there was something like a centralized control center for autonomous vehicles (rife with opportunities for system-wide hacks and corporate/gov't abuse) or if vehicles were networked and shared travel vector and size data.