The categories are gender segregated. It's not that no woman would ever do so or want to; it's that banning a woman from women's sports is saying that she's not a woman. That's the very point of the category to begin with. Forcing a woman to compete in men's category is declaring her to be a man, which is something the overwhelming majority of women would not stand for.
Recall the controversy surrounding Caster Semenya. She was very much declared not to be a woman by an international sporting organization.