Which is why they're usually just temporary visitors to Earth. The planet is actually passing through them, so they fade in, crying their despair at their lonely fate, and fade back out...
So…we’re flying through a universe filled with ghosts and the appear as we happen upon them as we glide through space. Which is truly fascinating since most ghosts people see appear to be humans or humanoids clad in local clothing from the most recent three centuries of Human fashion. It’s mind boggling to think how many recent-development earths must have passed just in front of us over the billions of years of universe for us to see such spirits.
Either that or there are a lot of liars and hallucinations out there. I know which one of these options I believe in!