I think it's to do with the number of layers of paper -- that is, you fold a piece of paper in half and you have 2 layers, fold it again you get 4, and so on. By the time you have fodled it seven times, that's 128 layers of paper -- which would be like trying to fold a small book in half! (I have heard it's possible to fold paper up to nine times -- 512 layers -- phew!)