How is a soul different from the living creature that it came from?



Short Version

If someone capable of casting a spell like Plane Shift dies, what prevents their soul from simply casting the spell and returning to the Material Plane?

Explanation

From what I can tell, when a humanoid dies in the Pathfinder universe, he or she usually goes through this process:

  1. Sometime after dying, his or her soul appears on the Ethereal Plane near the River of Souls.
  2. Over some unspecified amount of time, the River of Souls carries the soul to Pharasma’s Spire to be judged.
  3. Pharasma judges the soul, usually sending them to an appropriate Outer Plane based on their alignment. The souls of PCs are usually held in the Boneyard until they can be raised from the dead.
  4. Souls that pass on to the Outer Planes eventually become petitioners, losing their former abilities and their connect to their mortal body.

I can’t find any information about how the soul is mechanically different from the original PC. However, the description of petitioners (including the Petitioner template) makes me think that until the soul becomes a petitioner, they’re basically the same as the original creature. This seems consistent with other D&D-related media like Baldur’s Gate and Order of the Stick, where the souls of the dead can retain class abilities and maybe even some gear.

If the soul is basically the same as the original creature and still has class abilities, what stops a caster from resting for 8 hours and then casting Plane Shift? How is this different from being brought back by Resurrection (other than saving 10,000gp)?