Why are there no good X-Box 360 games for girls?

I have an X-Box 360 and my brother has the games Halo 3 and Test Drive: Unlimited. I've tried to find games that I would like to play, but there are none. Why don't any game companys make good girls games for X-Box? I was guessing because X-Box is a guy's system, unlike Nintendo systems like the GameCube or Wii.