I think the reason is is that we're social cognitive primates, and that our fundamental a priori cognitive categories are masculine, feminine, and child. That's the fundamental structure of reality, because we're social creatures and we view reality as something that's essentially social in its nature. And then when we started to conceptualize reality outside the social world (which wasn't very long ago by the way, and which is something that animals virtually don't do at all) we use those a priori social categories as filters through which we interpret the external world. And we're sort of stuck with that in some deep sense.
And you might say well why do we have to be stuck with that. It's like well because some things are very difficult to change. Like if you go watch a story, and the characters in this story slot themselves into those archetypal categories, then you'll understand the story; and if they don't, you won't. Because your understanding is predicated on an application of the archetypal a prioris to the story. You wouldn't understand it otherwise. So you can't get under that, there's no "under" that, and not remain human.
So I can give you a quick quick example. I like to use Disney movies, for a variety of reasons, mostly because everybody knows them. The evil queen in Sleeping Beauty is not an accidental character — she's the way she is because we understand her and the reason we understand her is because we see the world through the categories that I just laid out.
Are you saying she has to be a queen and not a king?
No, if she was an evil king, she'd be different, she'd be like Scar in the Lion King. He's just as evil, man, but not the same character, right?
(I think he doesn't understand the is-ought problem.)