The main rotor hit his head -> Because it suffered a material/design/ structural failure -> Because design validation or material quality checks or weld quality NDT checks or bolt sizing or balancing or lateral vibration suppression or a million other things were not checked -> Because you need to know to do that, and those are taught in an engineering course.

Even if you know that, failures still happen all the time because as you state, there are thousand things that could go wrong and this task could be too much for a single person.