
Originally Posted by
WellGramarye
Souls come from the Aetherial Sea. They're a natural product of the world's processes, and just come into being eventually. Creation magic was unable to form them.
From Through his Eyes Side Story
This explains the natural cycle of souls, rising from the Aetherial Sea, and becoming new life and how the Ancients couldn't manipulate it.
Emet was staring up at the sky and could see souls in the aether currents around the area, that would then fall into the Aetherial Sea, which they called the Underworld.
This part re-enforces the previous bits.
In Tales from The Dawn, A Friendship Record we get this part :
Zodiark as Emet put it during the Amourot dungeon could "weave the laws of nature anew," meaning it could manipulate the Aetherial Sea, and actually create souls en-masse from it. This is the "new life" that never knew the End of Days, from which most of intelligent life as we know it descends. The Sundering stopped the harvest of this life, and split all souls.
We can infer three things:
1. Those who were alive and Ancients at the time of the Sundering, became those whom have the potential for the echo.
2. Those who were created though Zodiark, are those who do not have the gift at all.
3. Those were Ancients before but were not corporeal during the Sundering, feel the nostalgia and sorrow, the loss of their civilization.