Because you would be risking nothing and markets don't work this way. And everyone would be doing it and then there would be no spreads anymore.
so why isnt everybodt doing it?
Because you can't sell something you haven't bought yet. And short selling I'm aware of, but this is different.
Yeah it might be possible with few exchange sites. Most websites don't allow users to do trading till they have the respective amount within their wallet connected to the trading profile.