This is the explanation closest to what I was thinking beforehand. The problem seems like one of the difference between {the difficulty of predicting an event} and {the likelihood of correctly reporting an observed event}. I think Dagon's argument about Map vs. Territory is a good one too.
Question for you, though... please define "ghost"? I have a feeling your definition is different than mine because I find events such as
certain environmental factors like (low-level poisoning from radon, carbon monoxide, et al; certain acoustic effects; certain architectural events such as uneven expansion due to temperature changes; &c.) cause minor hallucinations or illusions resulting in supersocial minds like those in humans perceiving "people" where there are none
very much more likely than
something of a person that exists independent of the usual corporeal form and (typically) despite the loss of that form is detectable by an uninformed and objective observer.
EDIT: formatted for better readability
I was joking about ghosts... :)
It's a good point, though. I think minor hallucinations and illusions area much more probable explanation for ghosts—and lots of other alleged paranormal/supernatural phenomena—than anything authentic.
Alice: "I just flipped a coin [large number] times. Here's the sequence I got:
(Alice presents her sequence.)
Bob: No, you didn't. The probability of having gotten that particular sequence is 1/2^[large number]. Which is basically impossible. I don't believe you.
Alice: But I had to get some sequence or other. You'd make the same claim regardless of what sequence I showed you.
Bob: True. But am I really supposed to believe you that a 1/2^[large number] event happened, just because you tell me it did, or because you showed me a video of it happening, or even if I watched it happen with my own eyes? My observations are always fallible, and if you make an event improbable enough, why shouldn't I be skeptical even if I think I observed it?
Alice: Someone usually wins the lottery. Should the person who finds out that their ticket had the winning numbers believe the opposite, because winning is so improbable?
Bob: What's the difference between finding out you've won the lottery and finding out that your neighbor is a 500 year old vampire, or that your house is haunted by real ghosts? All of these events are extremely improbable given what we know of the world.
Alice: There's improbable, and then there's impossible. 500 year old vampires and ghosts don't exist.
Bob: As far as you know. And I bet more people claim to have seen ghosts than have won more than 100 million dollars in the lottery.
Alice: I still think there's something wrong with your reasoning here.