Cars don't have the equivalent of air traffic control that will at least attempt to alert an off-course airplane, and if they can't manage that, divert other traffic, as they do in any emergency. With cars...it's a wild world out there.
And here we have a big part of the problem. People (perhaps rightly given the discussion and linked video in this thread) perceive auto pilot as capable of controlling a great deal of the flight with the crew doing very little much of the time (there are, I believe, even planes (fighters) that humans could not directly fly but require a flight computer to manage most of the load). But we fail to make the critical connection - the skies are well managed with lots of space between aircraft (and by necessity - airliners cover a great deal of ground in a few seconds and if a bit falls off you can't simply pull over and stop!) where as the roads, well.. Any idiot can get on them at any time, and a great many do.