Airplane autopilots really are solving a much simpler problem than even a level 3 autonomous car has to solve.
The most advanced autopilots can automatically takeoff and land, but that requires substantial ground equipment at each airport to support.
You say that it would take a massive investment but realistically existing signage, reflectors, and road markings probably cost more than the equivalent for an autonomous vehicle would.
The exciting thing about self-driving cars this time was that they worked on existing roads, making them financially feasible.
Perhaps final stages of ILS approach? That'd be the exception, but pilots are trained and certified for this - and definitely not allowed to snooze off then.
(I'm not a pilot, but have spent a fair amount of time and money on various PC flight sims; edit: and used to drive a Mazda 3 with a very good radar cruise control and AEB, before ditching it for a bicycle because getting old and fat).