To those that usually stick to one archetype of character throughout most RPGs-

[Alxyzntlct]

Well, I personally do exactly what you described because it comes from a feeling deep within me, a personal inspiration / desire / urge that is always pushing forth, something that simply radiates out that is never ending. I don't really know why, but I've certainly realized the trend over the course of my life, lol

To provide a bit more detail:
I've always focused on spell-casters in games, especially anything along the lines of a Black Mage, or if available, a Multi-Caster. The best example of my greatest ideal in a spell-caster was in Final Fantasy Tactics, actually, specifically the Arithmetician (aka Calculator back in the day). The thing I loved about that class/job in that game was that it was built up by learning all types of magic from multiple other Jobs, and on top of that, could mix/match abilities as well. All in all, it was a master of magic, a "manipulator of reality".

What drew me to FFXIV wasn't just being able to play BLM, but being able to all types of magic on the same character. So to me, even though I'll need to swap Jobs depending on what I want to do, no matter what magic I'm casting I'm still the same person. conversely, any of the more "physical" jobs I'll play invariably will be the more magic-oriented jobs because they fit that self image I have persistently hovering in my head, and is the closest thing I can get to the Arithmetician outside of FFT.

BLM was my 1st Job because it's the quintessential spell-caster, or "reality manipulator" borne from study and dedication (which is also denoted per the Ivalician raids which has the Arithmetician set for Mages, hehe). From there I delved in AST because to me the study of the stars isn't far off from the study of magic, and gives birth to a magic that has the ability to heal (which is a role I also thoroughly enjoy, plus Astrologian is also a call-back to FFT). After that, I dived into GNB because to me, it's effectively a Mage Tank, lol. The job is focused on focusing magic into bullets/shells, which then can be localized to output incredibly powerful offensive and defensive magics.

The only physical ranged class I'm considering is a BRD, because even though it does use a bow and arrow, it uses magic to instill greater power and influence / support ones allies. As for a melee job, I'm considering NIN because past the rogue stage (which I care little for), it expands a great deal into using Ninjitsu magic (which I'm a big fan of).

Outside of that, while I'm kinda interested in leveling up the rest of the Jobs because I'm interested in the story, I doubt I'll ever main them because they don't feel as tied to the inner sense of self I have...

As for the handful of jobs I feel I will focus on, I will dance between all of those because at varying times I'm in varying moods; sometimes I want to run into the mosh pit of a fight and bash things around as a tank, protect my team. Sometimes I want to hover around the edges and provide support, healing allies and providing buffs. Sometimes I want to burn the world. While I would LOVE to mix/match abilities like FFT could, I get how they tried to do that with this game and had to stop due to the complex difficulties it created, so I do my best to be content with what's available.

So yeah, there ya go. I can't explain fully WHY I'm this way, I just know it is after years of experience and trial/error. Like, I know I don't do alts. I've tried in the past but always quickly lose interest, I invariably always swing back to my main shortly after the attempt. Maybe it's just some part of me wanting to find some way to be the me I want to be? I don't really know... but I can at least share the experience so you can make your own determinations.

Maybe the challenge you're having isn't just find a single Job to focus on, but possibly finding the Theme you enjoy. Although it sounds like you have a theme you like, now it's just a matter of finding the components that'll make you happy exploring that theme. What you mentioned about how the "Mage classes" call to you is pretty telling, so maybe it's not a matter of just picking a single Job to focus on but finding the various Jobs that fulfill the moods you have as they ebb and flow?