That is just deeply untrue. Even if automatic pilots had statistically less accidents than real ones, any accident caused by machine error would be greatly publicized, and manufacturer of the autopilot would have to pay $$$ in damages.
Let's use a different argument, just for the sake of it.
Let's assume there exists a medical device, which never fails on its own, but due to operator errors 1/10000 patients who use this machine die.
Now someone makes a fully automated machine,but due to problems with programming it kills 1/100000 patients it treats(completely on random). Even though it's 10x better than your average operator, such machine would be banned from the market quicker than you can say "litigation". In our society human errors are much more easily forgiven than computer errors, because everyone believes that they could have been prevented. And therefore, a truck autopilot wouldn't need to be just better than your average driver - it would need to be absolutely perfect. And I don't think we are ready for perfect yet.