Interesting topic.
Ghost writing, or co-writing, did not evolve for a non valid reason.
Experiencing something does not translate into being able to expertly describe that experience. If so, there would be far more bestselling autobiographies- with no co-authors.
There are some bodybuilders who lack sufficient verbal and/or written skills. Were their unedited and uncorrected offerings printed as they alone composed the words, well, we would then be observing the reason that editors became necessary, wouldn't we?
Some actors, when working without a prepared script- such as when they are being interviewed live, reveal how non-fluent they are when out of their element. The writer cannot perform the function (usually) of the bodybuilder (or of the actor), and usually the only cablegram the bodybuilder can elevate is on a pulley machine. And, shall we say, not a crossover pulley?