It's not that you can't have people who are inherently conscientious (heck, it's considered a personality trait); it's rather that it's very hard to filter the non-conscientious people out by anything other than attrition, because in conscientiousness-rewarding industries, the non-conscientious have every reason to pretend to be otherwise.

With the checklist analogy: some doctors are willing to follow checklists; other doctors aren't. Of the doctors who have killed a patient from not following an available checklist, usually all are willing to follow a checklist—for the ones that still aren't, and kept on killing patients, have been disbarred. "Did a bad thing and is still a doctor" is a sort of survival-bias heuristic for filtering out the non-conscientious.

Here's another way to think about this: a soldier might have trained for years and years, and be very good at all the arts of war. But most people would prefer to staff a "hand-picked" strike force with veterans—soldiers who has been in a number of battles, and in particular have been engaged in a number of individual combats—rather than just people with "natural talent." Because attrition ensures that the only veterans who survive are the ones who know their business.