Also consider the possibility that their training habits may be so poor that it's actually working against them along with a poor diet and inadequate rest and that the truckload of androgens is the only thing serving to keep them just above water. Many times these are the people that start training and 3 months later just look worse and feel terrible and give up.
All the while they watch another guy come in 2 maybe 3 days a week, less androgens and throws some weights around for a while but then goes home and gets adequate rest and eats good foods. Then they'll say he has great genetics but really it's just that he put some stress on the muscle but had other key components right. While original guy had more than enough androgens and anabolics but absolutely nothing else right - overtrained, malnourished, no rest.
A lot of stars have align to make a person a candidate for growth. AAS and PEDs is just one link in a long line of so many things. Genetics, training, diet, rest, chemicals (PEDs). Training knowledge, training wisdom, and common sense creates muscle combined and proportional to genetic ability. At least that's always been very much my opinion.
Genetics definitely has a big role in it all. But me personally, I try to be careful with that word because it can also be very easily used as a copout. And I think it is more often than we think.