Why isn't ATP available as a weightlifting or dietary supplement? What am I missing here?

Most of these supplements like creatine, carnitine, glutamine and some of the BCAAs work by increasing availabiliy or enhancing the your ability to use ATP. Why not skip the middle man and just give you ATP or some other high energy compound that could be used by your cells to aid our muscles in respiration? I have wondered this since AP bio which was like 11+ yrs ago now for me. Wouldn't taking ATP make you a weight-lifting beast? There must be something I am missing b/c GNC would not hesitate to market this if there was any chance of turning a profit. Who knows their cellular bio and/or biochem?