Seems to be same for most action games. There’s probably plenty of reasons:
For one, it’s because in real world, many enemish occupations like gangbangers, police and soldiers are predominantly men. So guess it just doesn’t occur to makers to make female enemies to fight aside from bosses.
For two, it would require additional models and voice acting. Most devs are probably too lazy for that.
For three, my bad if I’m wrong but shooter games are also still predominantly male field. Guess makers assume that we men just don’t like shooting women. Or burn them with fire and acid and hear their screams of agony. Also while, f.e. Saints Row the Third had female enemies, I for one found myself more than once checking out them well-stacked female Morningstars’ corpses. And then felt like a creep about it.
But personally I wouldn’t really care if we had female mooks in the future games. When they shoot at me, i think about shooting back and not some old-fashioned chivarly code.