I think most roleplayers get this idea that the jobs are specifically harder to get due to the way they're setup, but also due to the way RPers approach wanting to try to perfectly mesh being a young character in the world in a relatable way. You see tons of overlap and such between RPers, because let's be honest, when RP is communal people often steal/take/lift/bite/borrow/blend ideas they see other people do. It results in flavor of the month sort of characters that change every few months to whatever's in style.
That aside, you just have to realize that RP is essentially writing. The limits to it are tied to your imagination. While the game does like to make proclamations such as, "This is the last scion of the noble Allagan house of pantaloons, and you just so happened to be able to master it and kill off the only remaining heir that could possibly master the technique!" It is also just as quick to dismiss or retcon such notions with things such as, "Character X was presumed dead by Ye Olde RDM mentor or Ye Olde BLM mentor, but they weren't. They were just missing on the other side of the world, and hey, guess what, they taught the super special snowflake unique arts to some chick you see at a cursory glance in Ye New Bozja content."
This absolutely gives you the freedom to do the same. It's a wild and fantastical world, and until something concrete completely contradicts your idea, then you needn't give what seem to be complications a second thought. A lot of the world in FFXIV will forever remain a mystery so that the devs can continue the game for as long as it prints money, which ain't happening any time soon!
Some ideas just off the top of my head, for even the lost/special/one and onry types of jobs:
SMN - Character winds up being the son or daughter or what have you of an Allagan Clone who went off rails and managed to reproduce naturally with a loving man/woman, passing the art onto their offspring, who inherited the same primal residuals via genetics.
SCH - Every technique comes from somewhere. Someone has invented every job. One such Arcanist fond of butterflies and knowing of Fae style myth delves into "new" aetherial geometry, creating their own fairy/pixie. Later on they learn they weren't the first person with this idea. There's nothing new under the sun.
RDM - An ex-Crimson Duelist, thought slain by the traitor, instead he clung stubbornly to life, devoid of memory and magicks for no few years. Time heals most wounds, and certainly ends them all, so one day this person finds they know a spell called Jolt. Over the proceeding moons, more follows, and soon they've a small sword and catalyst in hand once more, ready to strike out to find what became of the other RDMs and the traitor.
DRK - "Do you truly think that Ser Ompagne Deepblack was the first person whose heart burnt out of time with righteous indignation for all those suffering under oppression or betrayal? The first person whose aether swirled and churned from a burning black heart? The only one ever moved to possess a technique allowing one to move their broken body in spite of any missing ligaments or sinew? Then you're a fool who's ignored history. I know not where my mentor learnt the arts, nor where he found these soul crystals, but he did and now I do, and neither of us are Ishgardian." - A large Hyur, all in deepest black, wielding a sword that's less a sword and more a giant slab of steel.
I could go on, and while I wouldn't(and don't) have the best examples for how to do this, my point is that if you can think of a way to write it in, you can make it make sense. There's a lot more room for this kind of stuff than a lot of RPers think. Particularly because the NPCs we encounter in the game are not anywhere close to all knowing. They are frail and fallible and certainly not the lore keepers or masters.