The usual complexity of value considerations. The meaning of the stories (i.e. specifications detailed enough to actually implement, the way they should be and not simply the way a human would try elaborating) is not given just by the text of the stories, and once you're able to figure out the way things should be, you no longer need human-generated stories.

This is a different kind of object, and having lots of stories doesn't obviously help. Even if the stories would serve some purpose, I don't quite see how waiting for an explicit collection of stories is going to help in developing the tools that use them.