I agree with Lemon - societal acceptance and culture plays a HUGE part.
We live in a society with hundreds of years of ghost stories and programmes like 'ghost hunters' and Derek Acoras ghost programmes (where he was later exposed as a fake) so it's embeded in our culture that 'we don't really know, maybe there's something else'.
Only a couple of hundred years ago we were open - minded about werewolves, vampires and witches. We executed people because of it and dug up deceased people to drive stakes through their hearts. I've read contemporary accounts of exhumations where 'fingernails and hair was growing and there was evidence of freshly consumed blood'. I have no doubt those people believed that.
In Haiti there is still a belief in zombies. In many cultures curses, evil spirits, witchcraft and demonic possession are believed.
But not in our culture so if people were on here saying 'my neighbour has cursed me' or 'my child is possessed' or 'I saw a zombie' we would feel very, very confident in saying 'no, that's impossible, it's something else with a natural explanation'.
But with ghosts, because of our culture and our progression over time where we realised vampires and werewolves weren't real but ghosts MIGHT be. It's still offered as a possible explanation.
I think over the next decades/centuries we'll think of ghosts in the same way as vampires and werewolves.
Of someone came on here and said 'I saw a unicorn' we would be 100% sure in saying there's no such thing, they don't exist. But as our culture says ghosts might exist we have that doubt.