Why not ˚data-role="popup"˚ outside of ˚data-role="page"˚ ?

Why not ˚data-role="popup"˚ outside of ˚data-role="page"˚ ?

i am wondering why the designated popup content element data-role="popup" needs to be inside of the data-role="page" element. wouldn't it make more sense to place it outside of it, since it is not actually part of the page displayed, but an extra element that shows up later on during runtime.