It really shouldn’t be inconceivable. The guys who are so successful in bodybuilding are that way because of having the genetics for it physiologically as well as having the personal drive and desire. That’s no predictor at all of interest or aptitude in esoteric use of pharmaceuticals.
Which does the average sensible person do, assuming let’s say he can obtain any pharmaceutical drug without prescription: spend years learning himself what ought to be prescribed when and then self-medicate, or does he figure, Another person, an expert, has already learned all this, he has a good reputation, I will find out what he says is right for me and do that?
Now, granted, steroid “gurus” are’t doctors and there have been cases where trust wasn’t deserved, but in the situation where a bodybuilder does for whatever reason trust a given expert, it’s completely reasonable for him to not choose to worry about the specifics.
Myself I have always used the method of providing the understanding and explaining choices rather than simply saying “Do this” but I wouldn’t be surprised if there are many that leave it at “Do this” and have clients perfectly happy with that.