A self driving car needs to be able to reliably drive within ~1 foot of arbitrary obstacles, recognize junk in the road, recognize people in the road, obey vague hand signals from random cops, obey every weird traffic sign in the world, obey every traffic law, and disobey signs and laws when convention requires it.

Helicopters don’t have to deal with any of that. There won’t be any random obstacles. No human drivers to contend with. No conflict between legality and practice to navigate.