The Planet of Grotia was a very special one and of an incredibly rare type; rare to the point it was common to not have such a planet in an entire galaxy.
There was a special phenomenon that happened on other planets —very rarely— that when someone died, due to strange circumstances, their souls remained on the planet, waiting to find a way to leave.
That sort of event was incredibly rare and was in general what gave birth to talks of ghosts and spirits on most planets.
However, on the planet of Grotia, this was not rare at all. When someone died, there was a low, but definite possibility of someone remaining within the planet.
It was low enough that it didn't happen all the time but high enough that 1 in 50 deaths resulted in the creation of a ghost, or what the people of Grotia called a Spirit.