Y'all ever think about how Spiritomb isnt just bound to any old rock, but to a keystone specifically? For those who don't know, keystones are specific stones used in the tops of stone arches, like in bridges or doorways or such. This is backed up by Spiritomb originally being found in the ruins of a tower in Sinnoh.
This means it was tradition to specifically seal Spiritomb in archways. If I remember my architecture right, keystones had to be inserted last, as the two sides of the arch push on it in order to keep it in place. Whoever bound Spiritomb in the past saw fit to specifically build these villainous souls into structures, into the most pivotal part of a structure. Was it to keep them contained, so as long as the building stood, they'd be imprisoned? Was it punishment, always putting pressure on them? Atonement, so the evil could do some good in the world by helping the living? Or were they harnessed as wards, the souls of the damned harnessed to scare off other foul beasts or demons?
I dont know. What I do know is that Spiritomb is bloody awesome.
(And note: wasnt there a Spiritomb in Almia that could move throughout a ruin, sprouting from the floor rather than a stone? Can a Spiritomb be bound to the entire structure its keystone is used in creating?)
