> Do not need to deal other non autonomous devices while in flight.
In a decent-sized city, there will be low-flying helicopters for police, fire and medical emergencies. Admittedly, this is much less common than for autonomous cars, but any slight problem could easily become catastrophic (no such thing as a fender bender in mid-air).
> Do not need to follow existing road traffic rules.
But do need to follow existing air traffic rules. New regulation would also certainly be drafted (one would hope anyway).
> Lot of paths available.
Maybe, again depends on regulation. Commercial airplanes do have certain "lanes" they are supposed to stay in, and are not allowed over certain areas.
Benefits of Autonomous drones:
1. Do not need to deal other non autonomous devices while in flight. Cars needs to deal with human, other cars driven by humans.
2. Do not need to follow existing road traffic rules.
3. Lot of paths available. Cars are limited to existing roads. Drones can have a lot of flight paths.
I understand that there are other challenges - more energy required, less safe in case of an accident, flight regulations.
But still. It seems like if the technology is there, it would be an easier transition with Drones vs Cars.