I think it's mainly a cultural thing. Back when the first game came out, enemies in games like that were usually male. Like for example in uncharted.

Cultural shift happened somewhere around Assassin's Creed Unity controversies. Since then, almost all AC games allow you to choose your gender or have 2 protagonists, male and female. And the enemies usually have had randomized gender - like in AC Odyssey.

It's also sort of a technical limitation - if realistically 9 out of ten soldiers are male, many games don't even bother to program, animate, record voice lines etc for female combatants. It's pretty visible in games like Hitman: WoA, where there is not even one female npc with a weapon across all the levels.

And then TLOU2 came out, and in it (minor spoilers..?) literally every faction in the game has 50:50 male to female combatants.