Real time ghosts would increase the chance of pinging hugely; online it's the servers that handle the physics calculations so you'd have to be constantly sending/receiving data for it to work - even the slightest blip would corrupt the move or cause a player dropout. Also keep in mind that ghosts can't predict DM, fracture & other impact events that are handed as they happen.
(even normal hits aren't correctly displayed by ghosts)

Think kinda like what happens if you join a server mid match & the replay spazzes out, except with added ping.