have you ever seen the sky?
the same reason you haven't (i believe..) ears in the upside of your head, is why we don`t like sound reflections (positional information) from upside.. mankind, and before, have lived in open spaces by many time. Ground and sides offers valuable info, but usually you haven't too much positional information coming from upstairs... forget my english; I like your site very much. Wonderfull information, well explained too. Good Luck.