One factor here is that the people running Hollywood studios assume that men don’t want to buy tickets to a movie where the main female character is anything but the romantic partner of the main male character. (Yeah, the success of the Harry Potter series demonstrates that this assumption is bullshit, but... well... studio execs have a lot of practice rationalizing away exceptions like this. It’s like their assumption that white audiences won’t watch movies with a black main character, unless that main character is played by Will Smith.)