I found this YouTube clip of Joss Whedon's acceptance speech for Equality Now in 2006 where he talks about why he feels strong female characters are important.
In the end, it's a simple answer: because there aren't enough of them and people are still surprised by them.
But his impassioned statement gets me every time: "Because equality is not a concept, it's not something we should be striving for. It's a necessity. Equality is like gravity. We need it to stand on this Earth as men and women. And the misogyny which is in every culture is not a true part of the human condition. It is life out of balance and that imbalance is sucking something out of the soul of every man and woman who is confronted with it. We need equality, kinda now."
Only Joss could say something which strikes so deep and so true, and yet still funny.
I believe he's right. Misogyny and hatred aren't our birthright despite a certain universality. We're an aggressive species. We managed to destroy our other hominid relatives (or at least the paleontological record strongly suggests we did). Our determination to shape the world into what we wanted has been both our greatest asset and our greatest weakness. But maybe also our greatest hope. Because we still have that determination.