Because in the real world, people won't exercise exactly the same level of situational awareness, such that the autopilot only adds a safety mechanism. In the real world, it will replace a certain amount of driver attention. And the question is, is it always superior to human attention?
It's probably net superior, but I bet it will cause a couple of high profile accidents that a careful human could have avoided, and then people (or the government) will be scared of it.
The more valuable question is this:
Does it perform better than humans in the most frequently occurring situations that lead to incidents, are those situations a large enough fraction of all incidents such that the number of overall incidents drop significantly, and will that drop be greater than the increase in frequency of less predictable situations.
If the above turns out to be the case, then there will be fewer accidents despite less aware drivers.
Keep in mind every car out there is currently built with a feature where you can accelerate to a lethal speed, then press a small button, and the car continues to travel at that lethal speed until the car runs out of gas, hits a brick wall, or the driver consciously disengages it. We call it cruise control.
If all we are doing is adding automatic braking and lane following to cruise control, I don't see how it can be anything other than better.