Because that’s what this place was ... a repository of wandering, aimless souls. The grave site was densely haunted by them. Celesca Lovegood had picked up on that during her reading in the Ritual Chamber ... and now Harry was here, he could actually feel it for himself. Trapped by the cruel containment spells embedded into the security wards, the souls of the dead had nowhere to go. Murdered, then buried without any sort of funeral rite performed, they had no idea how to cross over to the realm beyond. Many probably didn’t even know that they were actually dead.