" why isn't it a feature of the iPod Touch?"
Because Apple chose not to make this a feature of the ipod touch.
Unless Apple announces, there is no reason to know why they do anything.
Does "why" really matter? It is NOT a feature of the ipod touch. It doesn't have a bottle opener either. It could, but it doesn't.
You can leave feedback for Apple at:
http://www.apple.com/feedback