Because a ghost revival is literally puts you back together piece by piece, well atom by atom. You could be a smear on the wall and your ghost could put you back together. It puts us back exactly how we are before we died. So, the revival at the very start of the game would be like waking up from a long and deep sleep - we'd be kind of weak and a bit stiff but other than that perfectly fine. And assuming we were very fit before we died we could be good enough to run fine almost instantly. [spoiler]Moreover if you're an exo I guess you don't really get tired [/spoiler]