^Again, I think the writers' actual record proves that they are more than capable of writing women as more than just eye candy. Evidently you haven't seen the shows and movies I referenced: I have, and I guarantee, the female leads were very strong and central to all of them. So you're absolutely wrong to accuse Abrams, Kurtzman, and Orci of sexism.
And I gave you a plausible reason -- pressure from the studio to conform to the target demographic. Surely you're aware that the creators of film and TV are not given total creative freedom, that they have to conform to the wishes of the studio or network executives. A lot of the decisions that shape films and TV shows are made by the executives whose names we don't see rather than the writers, directors, and producers whose names we do see.
Of course I have seen some of those shows and I do applaud those shows for having female leads. Buffy and Xena weren't the first shows to have female leads but they were among the first modern shows to prove that such shows could be phenomenally popular. I'm guessing here but I also expect that if you were to dig a bir deeper into the recurring cast you would find a ratio of 2:1 male:female. That's not terrible but it's not equality and it's also far, far better than the ratio we have in NuTrek at the moment.
I think probably Dollhouse and Battlestar Galactica did the best in evening up numbers and prominence of the men and women. Alias had a fairly decent mix too and some fun with its supporting characters as the series progressed but it was a vehicle for the luminous Jennifer Garner and she eclipsed everybody, both male and female - that woman could cry and change wigs in one breath. I wouldn't give Mission Impossible a pass though. The team that was wiped out in the first film was balanced but ever since then we've had no female agents in the implausible second movie, and only one female agent in each of the sequels compared to 3-4 male agents. The male agents have been varying in age, appearance, and skill, while the women are exclusively young and pretty. I freely admit I was gutted when Kristen Scott-Thomas didn't make it into any of the sequels - now that woman could have been an awesome IMF agent.