But most women have left. The men can't do that.
but as pp pointed out, they have left with their children. Plenty of women without children have remained and are fighting (and some women have sent their women off with family members and stayed to fight, keep the country running, etc)
And why is it that men have to stay to fight? Because historically female bodies have precluded women from serving their country on the frontlines, even though there have always been aspects of warfare (esp modern warfare) that don't rely on the brute strength that men have more than women generally.
I went into the Army many years ago. I actually wanted to join the navy because i preferred the uniform, the job i wanted to do was more to my taste (although all 3 services offered similar). But nope. Because in 1984 women weren't allowed on ships and that job was only done on ships. It wasn't even about "best person for the job" because when i went into the Army, the job had plenty of women, nearly 50/50 in fact. (and yes, some of it was because the Navy wouldn't have us). There was no part of that job that a man could do better just because his reproductive organs are on the outside.
Back to the topic: I'm a feminist because i believe in equality of opportunity. Which is why I'm a socialist (closer to communist but that's a big scary word for too many people). Women's struggles, and the class struggle is real.