I'm not sure why so many female writers write male heroes.

I think it's largely because it's drummed into us from childhood that men aer the default, the norm. Men are Interesting and have Interesting lives, while women's interests are small and narrow. I don't agree, but I think a lot of writers, who might not really have reflected to why their ideas always are around males, kind of fall back to the male POV as default. Or possibly to get more readers. I know this is true for chilren, that boys largely read books about boys, but girls read books about booth sexes. I think that may hold true for adults as well.

I think this is a bit better now, but as a child I was very annoyed that the few girls I could find, always had to be tomboys to be legtimate. I was a very girly girl, and it was disheartening that every time I found someone who liked the same things as me, she was always Not As Good as the tomboy heroine. It gave a very clear message that girls could only be as good as boys if they acted like boys.