I have not thought about it up till now but the whole series is based on the premise that when someone dies and has regrets and obsessions the be come shikabane to achieve them and kill in the process too, but what about the alternative to shikabane, or undead beings, why do ghosts not exist in this logic, I meant the logic of becoming a ghost to achieve your thing you wanna do before you passover is more believable than becoming undead, this is a big issue that is fighting in my head over the hypothetical logic of the paranormal, don't you think its interesting?