When a truck (or a train) can't see what to do on its own, it can brake to a full stop and remain motionless until a human intervenes. Cell network crashed? Mountain in the way of the transmitter? Somebody stepped on the cable? Whatever it is, most problems will be averted by just always braking to a halt and waiting.

This is not an option in a plane. If it was, this technology wouldn't be needed - if the pilot is incapacitated just brake to a halt and step out of the plane...