Correct. There's also the small detail that while autopilot systems might be a rounding error in the cost of an airplane, it would be a significant financial burden for average joe/jill driver.

Plus the sky is, on average, empty.

Can't we? (about the vast majority of automobile drivers)

Depending on where you set your standards, sure. But we have an expectation in the United States safety technology has to accommodate darn near everyone.

We have a law that airbags have to accommodate unbelted passengers: http://www.iihs.org/iihs/sr/statusreport/article/35/6/1

Now sure, a passenger can be different than the driver but it's the same philosophy.

The amount of illegal maneuvers I see every day on my commute is astonishing - not using blinkers, intruding on cross walks, not moving over for emergency vehicles, following too closely, etc. It doesn't help that the only traffic law enforcement is really around speeding / running red lights / DUIs.

The problem with autonomous cars isn't the autonomous cars - it's accommodating non-autonomous actors. It only takes one google car hitting an old lady who's chasing a duck on the street to become CNN breaking news for the next 3 months a la that airliner that disappeared.