When I'm driving I'm constantly making predictions about the future state of the highway and acting on that. For example before most people change lanes, even without using a signal they'll look and slightly move the car in that direction, up to a full second before they actually do it. Or I see two cars that are going to end up in a conflict state (trying to take the same location on the highway) so I pivot away from them and the recovery they will have to make.
Self driving cars for all I know are reactionary. They can't pick up on these things beforehand at this time and preemptively put them self in a safer position. Bad/distracted/unaware drivers are not only reactionary, they'll have a much slower reaction time than a self driving car.