lorewise there's nothing specific that explicitly prohibits or states that no asian-looking race currently exists or have ever existed in nirn.
in contrast, the akaviri samurai-looking weapons/armors and historical characters like "gaiden shinji", seems to be referencing to an asian-themed race and/or culture that currently hasn't been revealed to any TES game.... yet.
so due to this lack of representation, the devs are just likely to use other races as placeholders until they eventually add a new race or until they flesh out more the details surrounding the mysteries of the akaviri that could prove or disprove the existence of an asian-looking race..
-------
edit :
the ra gada of yokudan seems to favor scimitars, compared to the katana-like blades of the akaviri. so even though the ra gada have japanese-sounding names, the japanese theme of weapons/armors seem to be akaviri..
anyways, the lore behind yokuda, akaviri, ra gada, etc.. are still kinda shrouded in mystery.
from a gameplay perspective, i won't be surprised if bethesda suddenly decides to finally add a new race, yet at the same time, i won't be surprised if they don't add any either.
-------
edit 2 :
yokuda seems to be to the far west and akavir to the far east. if nirn is a sphere, then there's a possibility that yokuda and akavir could be neighbors. which means there's a high chance for crossbreeding and cultural exchange between the two.
meanwhile, some ra gada have more japanese sounding names compared to others that sounds more arabic, so it might indicate the presence of a different culture interacting and perhaps even crossbreeding with the local population of yokuda.
plus, scimitars and katanas are both curved swords. curved. swords.
so there might've been an intance in the distant past of redguards crossbreading with humans (or whatever they are) from akavir.
-------------
edit 3 :
lastly, pelagius septim III is a man/mer hybrid.. but iirc, he was depicted as a breton in-game (coz hybrids doesn't seem to be recognized in the game)
-------------
edit 4 :
oh wait :
-it seems that the "myth" that the tsaesci devoured all humans in akavir might only be a myth.
-secondly, having "scales" might just be an aesthetic/artificial/protective implants. like tribal tattoos and piercings. making them look more "snake like".
-it's hard to crossbreed with a half-human/half-snake creatures especially if the lower part are snakes. it's like trying to screw with a mermaid in their half-fish form.
-tthere are also theories that the humans that the nobles interbred with in cyrodiil were tsaesci human-slaves.. but why would nobles wanna marry slaves and think it was a privelage? there were also instances that said they prized the physical features of the tsaesci hybrids. which generally points that either the tsaesci are quite beautiful or at the very least, quite exotic.
---------------------------------
edit :
-so aside from being powerful mounted warriors (like mongols) (i doubt snake-tailed tsaesci could even ride horses)
-tsaesci fought using daikatanas and dragonscale armors (dragonscales could make them look like snakes though)
-the tsaesci also seemed to be more thematically linked to feudal japans. at least when it comes to their aesthetics.
-the tsaesci live very long lives. so there's a probability that there might be some still left hiding somewhere in tamriel, aside from their main population in akavir.
-another mystery is the amount of influence and the crossbreeding that the tsaesci might've done with the other humanoid populations of tamriel.
-as for WHY there are no signs/evidence of the presence of the tsaesci? that's the big mystery.
-but the tsaesci seems to be very smart and have great martial prowess and very long lived. perhaps this long life allows them to pursue strategies that extends throughout centuries. maybe they're just plotting another akaviri invasion and waiting for the perfect chance to do so.
-etc..
^there are so many mysteries left unanswered, so it could go either way.