The World Championship cycle is open to women. Hou Yifan partook in the Grand Prix. I believe Judith Polgar partook in a Candidates Tournament about a decade ago. It isn't segregated.

Nothing prevents a woman from becoming the World Champion. But, on the other hand, men are barred from the Women's World Championship.

Any explanation would also have to account for the genderless AIs that can be competitive at the international grandmaster level, given enough hardware power.