Insert Witty Journal Title Here

I think it stems from the fact that even when there is a female in these sorts of stories, she's the ONLY female. It's like when you have a group of characters, you have a hierarchy of roles to fill: fighter, nerd, healer, comic relief, and so forth. Now, not that a male couldn't fill the mother role, but if you have a female there she tends to get naturally slotted in. (Either that or girlfriend). If you add a second female you tend to get tomboy, and after two you've probably crossed the line out of tokenism and they are equally mixed into the general roles.

So... I think it's a problem with both assigning women stereotypical roles but also exacerbated by the lack of female characters in general.