I’m probably remembering wrong I haven’t watched these movies in a while, but weren’t the ghosts really hostile to everyone? I had the impression that other than superstition the ghosts really would kill anyone that approached them so nobody thought to. Aragorn was only able to make a deal with them because of some ancient family curse stuff he could free them from… which does sound machina-y, but it might explain why nobody tried to approach them up until that point