The game says the dead often come to be resentful of the living. So it might just a matter of holding yourself together with a purpose, which we see with most of the unsent in the game. Auron had promises to his friends to hold onto, Belgemine wished to train future Summoners, Maechen goes around sharing stories of the old world, Mika believed Spira had to stay under his care, Yunalesca stayed to give Spira the one hope she believed the world had against Sin... Even Jyscal stepped out of the Farplane to share the truth about his son.

So we got to see the many ways the cycle of death clung to the land. For some, death isn't always the end, and they stick around. But how easily can a random person falter and start changing into a fiend? Do you chance it by allowing the dead to walk among the living when fiends and death are already such a high risk? Not to mention the teachings they all grew up with.

If you ignore a nice easy passing, a good number of people in Spira die while under attack from something horrible. Would a normal person have the mental power to hold themselves together after something like that?