The answer to your question Crab.....Pilots are human beings and thus are fallible.
You ever been interrupted in the middle of a preflight....or run-up and missed something? If you have any experience you indeed have.
Ever skipped a line or two on a printed checklist following some interruption....again....if you have been flying very long...the answer is YES.
What we are talking about is a simple, mechanical, non-thinking, habit pattern that can prevent an UH OH from happening.
A simple thing like taking a wander all the way around the aircraft immediately before climbing into it....after doing a preflight...just to make sure.
Doing a hover check before all takeoffs.... or doing a clearing turn.....
Humans are interesting creatures....but are creatures of habit. Disturb one of those habits and watch the mistakes occur.
Ever sat in the rear of a simulator and watched aircrew at work? It is an excellent learning venue for the observer (sim instructor).