Why, then, Monks HAVE to be asianised things, or Samurai....
They don't have to be, I just thought I remembered someone in the early 4e stuff saying that if done, they WOULD be.
For the same reasons that clerics and paladins are very much Christian stereotypes with fantasy salad dressing (slaad dressing?), and stereotypical monks in anime worship the Buddha and use Ofuda, and the same reason I'd expect a Gladiator to be vaguely Roman-inspired, and a spear-wielding light-armored soldier or a serpent-themed oracle to be vaguely Greek-inspired. Or, get this, a squid-headed monstrosity to be vaguely Lovecraftian and a little bit out of the early 20th Century.
Regardless of how Arc Tan feels, our cultural gobbldegook is always impressed on our legends and myths, and that's where D&D draws a lot of inspiration.
Now, we can recast monks as generic "unarmed combatants," in the same way we can recasat clerics as generic "divine spellcasters," or paladins as generic "divine warriors," but I think you loose a lot of evocative potential when you reduce the class to functionalism alone. When I think of kickass martial arts, I'm thinking Bruce Lee, or Jet Li, or corny old Kung Fu movies. When I'm thinking of kickass samurai, I'm thinking Kurosawa and Musashi.
Why it is bad then to have monks in faerun, who is NOT europe or asia, by example? Or goblinoid samurai as in Eberron (suggested)?
Did I say it was?
I wouldn't say it is.
But Faerun has had a pretty strong case for being "European," and the goblinoids in Eberron have a pretty strong case for being "Vaguely Asian."
And vast benefits can be had from tapping the resovoir of mythological history that is inherent in the myths from which these archetypes are being drawn.
Heck, 3eOA even has a chapter recommending things like dwarven samurai.
So this isn't the fight you're looking for.