Think about what you said carefully.I've had some friends who thought getting into shape would finally get them a girlfriend, but what happens is that the women they get end up not wanting to get serious with them when they feel ready to commit.
When your friends were out of shape, they were not getting women. When they got in shape, they were able to get girlfriends but were not able to tie them down long term.
How did you reach the conclusion that being in shape is the problem. Their results actually improved when they got in shape which challenges your hypothesis and supports the argument their personality plays a role.