Asklemmy

Because they are old. Ghosts are just the anthropomorphic manifestation of people's fear of growing old. Religious framings are just an add on.

Trauma and grief can't run their course if your mind is so senile and your short term memory so feeble, that you're basically forced to live in the past. Forever repeating old arguments, reliving past trauma and never overcoming old fears. With your mind so set in it's tracks, that you can't even imagine leaving the place where you lived all your live — your "old haunt" so to speak. How could you live in the present, if you can't even recognize your own children half of the time? But the long term memory often still works. Ghosts are real and if you're lucky enough to live that long you might well become one. Of course aging isn't always like this, it can be graceful and dignified but when it isn't, that's what people are afraid of.

People are scared, when they see older relatives acting stranger every day, especially in times before any way to diagnose Alzheimer's and other forms of neural degradation. They might seem like they are not quite here anymore, like the person they were had long since died and yet, something lingers. Ghost stories are a socially acceptable way to express those fears.

Just observe the effects ghosts have on their victims: first, they are reminded of their own mortality. Then their hair suddenly turns white or gray or falls out, they lose sleep, wake up tired or grow old over night. They might lose their mind or die themselves. That's all just normal aging.

Here is a handy key to select monsters and their meaning:

• ghosts 👻: aging, death, old people
• vampires(folk believe): plague, infection
• vampires(literature): landlords, feudalism
• zombies(modern): alienation, capitalism
• witches: women who stand up to patriarchy
• Frankenstein's monster: the proletariat gaining class consciousness (no seriously)

I recommend the podcast "the horror vanguard" for details.