For the same reason that if I had a see-an-image-of-Grandpa button, and pushed it, I wouldn't count the fact that I saw him as evidence that he's somehow still alive, but if I saw him right now spontaneously, I would.

Imagine that you have a switch in your home which responds to your touch by turning on a lamp (this probably won't take much imagination). One day this lamp, which was off, suddenly and for no apparent reason turns on. Would you assign supernatural or mundane causes to this event?

Now this isn't absolute proof that the switch wasn't turned on by something otherworldly; perhaps it responds to both mundane and supernatural causes. But, well, if I may be blunt, Occam's Razor. If your best explanations are "the Hand of Zeus" and "Mittens, my cat," then ...