I think
@epronovost pretty much hit the nail on the head when it comes to deities. Anything can be deified so the question often isn't about whether or not they exist but whether or not a given individual sees fit to think of them as gods. When the deity in question is said to perform specific actions or possess a certain personality, I don't think it's unreasonable to say, "I haven't seen anything that would convince me that's true."
Ghosts does not exist, I never seen proof of them ( that I accept)
Ghosts are in a weird position in that I'm not convinced it's actually possible to prove their existence. I've heard some people say that if ghosts were real, we'd have pictures of them by now. My response to that is always, "Google it."
The problem with asking for photographic evidence of ghosts is that it's so damn easy to fake them but quite a bit trickier to definitively prove a photo is a hoax. There are so many pictures now that trying to determine whether or not each one is fake is genuinely impossible. By the time you've found one
definite hoax, hundreds of new pictures will have been uploaded on the internet. The end result here is that pictures or videos of ghosts just don't constitute proof anymore. Some paranormal researchers* have complained that it makes their job significantly harder and ask people to make them aware of any apps that add a ghost to a picture.
So if photos/videos aren't good enough, what else is there? The testimony of people who claim to have seen ghosts isn't good enough since it's possible that they're either lying or they hallucinated. You could perhaps measure the temperature of a supposedly haunted house to check for cold spots. Unfortunately you just can't prevent drafts and humidity in old houses, both of which can cause cold spots. Perhaps a place that causes an inexplicable sense of dread might suggest a haunting? Again, not necessarily. Faulty electronics (fans and air conditioning in particular) can create infrasound which may potentially cause feelings of discomfort. Alternatively, people might just naturally feel nervous in the kinds of buildings that tend to be associated with hauntings.
On top of all that, we're just
assuming that it's possible for a ghost to appear on camera, be visible to the naked eye, create cold spots and cause feelings of dread. Without a concrete idea of what a ghost actually is or whether it's a material being at all, we're fumbling in the dark in trying to prove their existence.
Does that mean it's unreasonable for people to not believe in them? No, of course not. Any of the things I just described constitute pretty valid grounds for scepticism. It does mean though that asking those who do believe in ghosts to provide evidence is putting them in an impossible position. There's nothing they can provide that can't be dismissed out of hand regardless of whether or not the thing they show is genuine.
*By which I mean people who make a serious attempt to study the paranormal. I do NOT mean those cretins who make reality TV shows.