I agree with your idea of leaky abstractions and overdriven goals. I think one thing to remember here is that while for sure attracting mates must play a role

I agree with your idea of leaky abstractions and overdriven goals. I think one thing to remember here is that while for sure attracting mates must play a role in the appeal of bodybuilding, there's also a competitive game aspect. You can compete against yourself and your peers. This can appeal to instinctive drives to compete and gain status, maybe again past the point where it's reasonable in some cases (are you really winning at life, gaining something meaningful by doing this?). If we just talk about the attracting mates aspect of bodybuilding, we also have to remember that if a man is trying to attract women by gaining muscle mass, his idea of what women in general might find attractive is just an approximation. Maybe this man found that he was gaining more attention from women when he started to gain muscle mass, so he just keeps doing more of that. Eventually he might gain even more attention for other reasons (improved self-confidence), but it gets hard to tease apart the real causal factors.