The onion children are explicitly implied to be untrained orphans granted power by crystals.

Realism can be fun in games as long as you don't lean too far in. "Sorry guys, I can't go on that road trip to learn more about the mysterious pendent we found last week. I'm reclassing and need to study for the next year to pass the bar."

I do think multiclassing or at least unrelated skills being possible is cool. Most medical doctors won't get to the top of their profession and then decide to retrain for years in hopes of becoming a seismologist, but they might teach themselves to play the tuba on the side.
True story. We have a guy at my work who falls asleep sitting up in meetings, wears t-shirts with dad jokes on them, and once insisted the office drip coffeemaker was "broken" when he actually forgot to pour water in before starting it. He's like 5th generation American but somehow also speaks like 7 languages. He's fluent enough in Czech(which isn't his ethnic background) he did sidework translating government documents for import/export businesses in the Czech Republic. How do we get that? He didn't reskill. He just picked up 6 additional languages unrelated to any of the professions he's ever had.
Doctor->Biologist->Veterinarian sort of makes sense as class path. Doctor->Circus Clown->Nun doesn't (I'm in the USA so correct me if it's different elsewhere). I think Doctor who also learns to weld or scuba dive, or some random skill from a group is closer to life.

Worth noting that they're not onion children in the remake. You start as a freelancer, and the first set of jobs doesn't become available until after you defeat the jinn. The actual Onion Knight job requires a mognet sidequest to access, which in the DS version requires communication with other players with the game (and doing it over wi-fi is no longer possible with the servers shut down).

As an interesting side note, characters in Final Fantasy 4 are described has having trained in their class; Rosa in particular is described as having trained to become a White Mage. (The only class change in FF4 is a scripted story event that's actually quite important from a plot perspective, but it only affects one character, doesn't give the player a choice, and only happens once. Interestingly, unlike every other class/job change in the series, the character starts back at level 1 (though with significantly more HP than anyone in the party has had up to this point).)

unless there is something in the narrative that forced a class change, then i prefer to not have it all. when i make a charcter, this is the character i want to play through the game. if I wanted something else, I would have made something else