Job Lore?

[Cilia]

I mean like, why each job exists and what each job was... well, what was their purpose in the past and why so few of them are, around (they are exceedingly rare compared to Classes).

PLD: Paladins are Sultansworn, the Sultanate's elite guards. Their organization is stated to be rather small.
DRG: Dragoons are Ishgardian soldiers, so DRG are rare to find outside of Ishgard. Specifically, you become an Azure Dragoon throughout the DRG quests, of which there is normally only one.
SMN: The art of Summoning was lost with the fall of Allag (I think). Either way, the villain of the Job Quests is a SMN going around and killing other SMN.
SCH: Scholars were Nymian tacticians before the Tonberry Plague. None exist in the present day.
NIN: It's an imported art, so there are few Eorzeans who know about and can use ninjutsu.

Now for the ones I only know about secondhand:

WAR: They come from Albathia's Spine in Ishgard's territory.
MNK: They're from Ala Mhigo, currently occupied by the Garleans.
BRD: ???
WHM / BLM: Both White and Black Magic were outlawed after they led to a Calamity (the fall of Amdapor - 3rd Umbral Calamity?).