I would guess it's because everyone knows someone who claims to have seen a ghost, or had an absolutely true reading from a psychic, it's sort of worked it's way into our psyche like that. It's part of our language.

Not many people have witnessed someone resurrected from the dead, or water turning to wine.

Even religious people have their own snobbery. If I went into church and said MTVN had performed a miracle, they wouldn't take me on face value, they'd want more proof than my word.