> It's basically just random chance that some planes every year will crash because of pilot error.

When you design a system where errors can cause a large number of deaths (airplanes, trains, nuclear reactors,...), your objective is to have very few accidents (nobody can guarantee zero). Having someone to blame when an accident happens -- the pilot did not follow procedure -- is not enough.

Your system design must be human error tolerant, because you know these will happen. To take an extreme example, if a drunk pilot error causes an accident it's not only his fault, but there probably also is a systemic error which caused this criminal behavior (which is a possibility you expect) to cause an accident: maybe the cockpit culture was such that the co-pilot was frightened to stand up to the drunk pilot.