I am currently reading "The Origin of Consciousness in the Breakdown of the Bicameral Mind" by Julian Jaynes. His theory is that ancient man was not conscious in the sense that we are conscious today. It's a fascinating theory and kind of hard to get your head around. If you believe that Jaynes' theory is correct then we see and hear ghosts (dead ancestors) because we are "wired" to see them. He doesn't say as much in the book but it's not much of a stretch.
Check it out:
Julian Jaynes - Wikipedia, the free encyclopedia
Bicameralism (psychology) - Wikipedia, the free encyclopedia
Of course, this doesn't answer why the existence of ghosts can't be "proven" in a material way. There are those who would say that our senses deceive us.
Personally, I'm a materialist. I don't think that the brain has "hidden" areas that pick up on supernatural "vibrations." That's me though. I could be wrong. It would be cool if I am wrong. The areas of the brain Jaynes' claims are responsible for bicameralism are pretty much well-documented (and have other purposes as well).