Yes, someone forgot to insert the bolts however the reality is mistakes happen and telling people not to make mistakes doesn’t work. You need to create an environment where mistakes don’t get anyone killed and management has failed to do that.

This is true in most engineering/technical/build industries. You need to create an environment where finding and preventing mistakes is celebrated and rewarded rather than penalizing those who make them. We're all human, errors are unavoidable. You just need to put systems into place to reduce, detect, and fix them when they happen. Ideally before a catastrophic failure.