Even if more screenwriters were women, I think there are certain assumptions in place about what The Audience (read: 16-35 years old and male) wants to see. And one of those assumptions is that women are interesting in a very small number of contexts: the Love Interest, the Token Girl On A Male Team, the Badass Action Hero (Look We Have A Strong Female Character!). And even a show like Avatar, which is very clear on Sexism Is Bad -- count the number of conversations Katara has with other women... I hear that there are actually regular female characters besides Katara in the second season, but so far it's surprisingly slim.