A big reason for the recurrence of jerks in fmg stories can probably be tied back to an author who went by Marknew. He was both very good at writing fmg, very prolific and was active for about two decades and counting. Maybe longer. All of his stories tended to feature role reversals (not uncommon in fmg) but what set his stories apart were his descriptions of the girl lording her good fortune over the guy. His girls weren't always abusive assholes, but in some of his more memorable stories they were. Over the years I've seen many writers enter this space who were clearly influence by him.
As to why, I'm in no position to psychoanalyze anyone, so I prefer to leave the motives a mystery