Prime Junta of course I know about the increase cost, even mentioned it. However the increase in exposure and sales, verses cost out weigh this, plus it's about changing industry standards for the better. If writers know from the beginning they are writing for avatars of both sexes then the cost factor is not that great compared to the increased exposure and sales. Additionally when your talking about theory cost is not a factor, as you well know.
Acleacius, no offense, but you really don't have a clue what you're talking about, and you're starting to look a little bit foolish by the way you're belaboring the point.
Specifically, creative writing is... creative. The more you know about the character that you're writing about, the deeper and more fleshed-out the story. Force the writer to think "if he's male, then this, but if she's female, then that; if he's gay, then this plays out like that, if she's a half-demon, then that happens" and the story will lose its coherency and personality. The only way to make, say, PS:T work for both a male and female protagonist would be to put two writers (or teams of writers) writing two separate stories with some coordination about scenery.
In other words,
to write two games with some similarities here and there.
With games tending to be released late and buggy even now, what sense could it possibly make to do that extra effort?
As others have said here, a game-writer has basically two choices:
(1) Make it possible for the character to be just about anybody, and write the story about other people, or
(2) Nail down as much as you can about the character, and make the story about him/her.
You cannot do both -- make a story that's deeply, personally involving, and make it so that your main character is a blank slate that the player will fill to whatever takes their whim. It's just not possible.