Higher protein/lower carb diet of lifters naturally leads to lower body fat, face and neck look better, which also correlates with reproductive success.
As if I needed yet another reason not to start smoking, I don't want to pass out coughing at the gym so I don't smoke, with a side effect of my teeth are therefore not yellow. Non-yellow teeth also correlate with reproductive success.
Even stuff like I don't binge drink or use drugs and get reasonable good sleep therefore my eyes are rarely bloodshot in the morning, again, eyes that don't look like a gore pic correlate with reproductive success.
A lower carb diet plus some sweating in the sauna eliminates all skin problems for me, and again lack of zits correlates well with reproductive success.
A final interesting correlation is there's an absolute fixation by non-lifters on "beating people up" whereas in reality I've lived in a concealed carry state for decades so people are much more civil and polite to each other and I'm not in a demographic or social class where beatings are part of life. I would instead propose it seems rather obvious that a guy who takes excellent care of himself will take excellent care of his partner and children, which seems to market pretty well to women, every woman wants to see her kids grow up healthy strong and attractive, so a guy that "cheats" his natural genetics by putting in a couple hours in the gym every week is going to massively attract women.
That said, the specific cause doesn't really matter, if you generally join the overall "lifter lifestyle", it will work out well. Eat better, exercise, treat your body well, and that will attract people.