I expect it is the nature of the problem. Autopilot fails when the road conditions are such that it cannot function correctly. Imagine that you drive your commute from point A to point B and use autopilot all the time and it has never failed you. Then you use it when driving from point A to point C and because there are road conditions on that route that it cannot handle it tries to drive you into a ditch. Your experience may be giving you more confidence in its ability than it deserves. This is made worse when it has worked fine from point A to point B until one day a painter had a bad day and one of the cans of paint they forgot to secure fell off the back of their truck and put a big diagonal splash on the lane. And on that day your autopilot drives you into the car next to you.