Because there’s a significant overlap between the AFC East Buffalo Bills lets go mafia shirt that cares a lot about a female protagonist and the audience wants to see romance. Not everyone of course, but enough that it usually makes marketing sense to combine the two. Same reason if you’re going to write a gritty war story, it’s usually some badass dude who doesn’t talk very much and has a scar on his face; the sort of people that like war stories tend to also be the same sort of people that like that sort of protagonist. Of course, there’s money to be made by bucking the trends and writing a war story with a female protagonist, or Mr. Badass McGruffguy finally finding true love, but that puts your squarely in a niche market. Publishing houses are usually looking for mass market appeal; they might take a risk on a niche story here and there, but they’re mostly looking for safe bets that will appeal to a broad audience, and that means sticking to the tried and true formulas.