Quote Originally Posted by Rocl View Post
It's not brought up as often any more, I think, but soulstones are rare (though becoming more common! think of all the soulstones the Summoner questline introduces and it's arguably the oldest job usable by the Warrior of Light!); so NPCs are often relegated only to classes, and so, the disparity.
Soulstones are not required to become a job (except SCH and WHM.)

Soulstones are a history of the job and the abilities learned by the previous holders, making it easier for the current holder to learn said abilities. It's possible to become a paladin/bard/dragoon/etc through time and effort and training alone. The soulstone is just a shortcut that lets you skip the years of extra training. (The exceptions being SCH where the fairy is tied to the stone, and WHM where the elementals won't accept a non-padjal without it.)