You could also ask why we have sexuality at all, and since we do, why isn’t it evenly spread across the spectrum?
Obviously there are mechanisms that guide our sexuality and gender identification, and they are not 100% the same for all men or all women.
It’s simply a natural mechanism with some variance that occur in most things.
Why are some people introvert?, Why are some people tall? Why are some people strong and others fast?

It’s like when you ask how magnets work, and it turns out they kind of work like everything else.
www.youtube.com/watch?v=Q1lL-hXO27Q