Board Thread:Lore Discussion/@comment-121.217.164.118-20130929050443/@comment-24902649-20141217084526

Dovahsebrom wrote: "and others find their souls unable or unwilling to depart due to unfinished business."
 * -Unhallowed Legions

The point being that there's a reason souls are still there if they have unfinished business. Otherwise Tamriel would be filled to the shit with ghosts who won't accept that they're dead.