And why many "real" self-driving systems that share the road with other users are often quite speed-limited, to allow them to come to a full stop relatively safely if needed, and give a human time to take over properly. (E.g. the Mercedes system will stick way below the usual speed limits, which means it's main use currently is handling heavy traffic with stop-and-go, or many of the autonomous "busses" for dense city spaces/campuses/...)
Passively monitoring a situation continuously and reacting quickly when needed is something humans are not good at. Even with airline pilots, where we do invest a lot in training and monitoring, it's a well-understood issue that having to take over in a surprising situation often leads to confusion and time needed to re-orient before they can take effective action. Which for planes is often fine, because you have the time buffer needed for that, but not always.
Passively monitoring a situation continuously and reacting quickly when needed is something humans are not good at. Even with airline pilots, where we do invest a lot in training and monitoring, it's a well-understood issue that having to take over in a surprising situation often leads to confusion and time needed to re-orient before they can take effective action. Which for planes is often fine, because you have the time buffer needed for that, but not always.