Skipping a day is useful for hypertrophy. Not so sure how useful it is for strength.

Depends on the strength program. Powerlifters often train just 3 days a week, with each day focused around one of the major lifts (bench press, squat, deadlift).

Olympic weightlifters train more often and work most of the same muscle groups each training day. They'll focus each training day around one of the two Olympic lifts (snatch, clean and jerk). I believe they usually train 4 days a week.

And then there are specialties like the Smolov Routine which has a cycle of intense squatting four days a week for three weeks. However, this level of intensity is not sustainable in the long run.

Hypothesis: I think the major body building or power lifting trainers like Rippetoe, who write the books and make the popular methods, have incredibly lot of willpower.

They're also the sort who'll come out of a hellish training session thinking "This is great," not "Never again". Willpower alone won't get you through unless you really think it's all worth it.