The title sums it all up. Why are pretty much every ninja i meet in games women? I don't have anything against it and you could ask the same question about why are all soldiers men, but that's not what i am asking.
Here are the games i know about in wich there are all these ninjas.
Mass Effect 3 (of course)
Half-life 2 (although that was cut) 
Saints Row 3 (kinda)
Okay that's not to many games but you get my point.