Because they are old. Ghosts are just the anthropomorphic manifestation of people’s fear of growing old. (et so on)
Damn, that’s some psychoanalysis you got there… that is a compelling stance on why it is so
Also, checking on the monsters, it makes sense, I tell you that… though I wonder who Frankenstein, the creator, reps, in this case
I could give a damn, but you know, it’s an overall race to the bottom