I think a lot of these are discipline strategies and I'd wager that personal trainers to professional athletes perform as less of an external source of discipline and more of an external brain/workout strategist/source of encouragement, as professional athletes usually already have the discipline/motivation a little under control.

Whereas personal trainers (as opposed to professional trainers and more in line with the previous comment's thought of a single nemesis/mentor) would need to be that external source of discipline, as well.

Totally guessing here, no experience in the training world whatsoever, feel free to tell me I'm way off base.