If Larian added a mountless Cavalier to the game, we'd see endless complaints about lacking the subclasses most defining feature.
Ok ... thats fair i gues.
It is still however, tied to Eastern locales in DnD. Which... Baldur's Gate is not. Sure, a PC could be a traveller from the East... But it's still an out of place subclass for the setting of BG3.
Just as Dragonborns are?
Just as Githyanki are?
Thats the thing ... as you said, people travel in this world.
And since we were obviously picked by Nautiloid "somewhere" wich was as it seems purposefully left vague, there is no limit.

And even if not, just bcs some subclass originate in eastern parts of Forgotten Realms, it dont meean they exists exclusively there and nobody else in whole world is allowed to become one, or invent the same thing on their own.

Its a Fantasy game ... dont be affraid to use it.

Both are focused around becoming large and gaining bonuses for it.
In other words they do one thing simmilarly.
I dunno man, feels like this argument cant stand on its own legs. :-/
If doing one thing simmilarly enough is a sign for abandoning one subclass ...
Then why do we have Eldritch Knight and Arcane Trickster? They are not simmilar enough?
And what is this big difference between basicaly all Wizard subclasses?

Not to mention that Elemental Monk is redundant aswell, since all he does is basicaly what wizard do, just worse and more expensive.

Yes, they get big ...
Thing is tho, that is not where it ends.

While Barbarian is throwing his two-hand weapon inbued with elemental power ...
Rune Knight is providing bonuses for whole group.
I just dont see the simmilarity here ... yes, they are both big while doing so ... but that is not really that much.

I dunno, if being big while doing anything is the same for you ...
Then Shooting Seeking arrow as Arcane Archer, casting Magic Missiles as Eldritch Knight ... or basicaly anyone else, since quite litterally anyone can have this spell ... should be the same aswell, no?

Storm Herald or Ancestral Guardian could have been interesting)
Now you are getting it!
Except "Range Ranger" is a playstyle choice, not a class one.
What is so "except" about that?

They still do the very same thing.
If your argument is that two things should not be too similar, i ask you what is difference here.
I see little to no difference thematicaly between Arane Archer and Eldritch Knight ...
Sure, they *CAN* be build differently, but as someone said not so long ago
that is a playstyle choice, not a class one. :P
Even their weapon focuses provides them with mostly melee options.
Yeah wich have nothing to do with the fact that DnD provides like thrice as much meele weapons, than ranged huh? xD
Which is reflected in all the Githyanki we face in game. Not all of them are Fighters.
I said subclass ...
If you are trying to pull out this "that is not a class" on Ranger ... then stay on it on Fighter subclasses ... be consistent. :P
"Being Cool" isn't enough of a reason to choose various subclasses.
Oh come on ...
Most of the time its the only reason. xD