Both autonomous cars and autonomous aircraft (likely) have manual backups. So the goal is to be 99% autonomous, or something like that. That works for flying because the failures are probably more far between. The planes are very well maintained, the route is clear from other planes etc. For a car, reaching 90, 95 and 99% is really difficult because of other drivers, bad weather, ice covered sensors, higher risk of mechanical failures. I agree making a 100% autonomous plane that needs to land safely in an emergency is probably more difficult for the reason you mention.