I don't actually own CK3, and I don't have any inside information from PDS, but I do know a thing or three about expanding the map further east in CK2 and have kept up with CK3 to such an extent that I feel I can give a decent answer to the question as to why it hasn't been done.
First off, you are going to need to add a lot of provinces on top of what already exists. How many you need will depend on which areas you actually want to include as non-wasteland provinces and the desired granularity, but you're probably looking at over a thousand counties assuming you're aiming for everything from Sakhalin in the north to Bali in the south (which, by the way, would mean you probably want to add a decent chunk of counties in Africa, and perhaps want to go even further south due to that), which naturally will need more than one barony in many places, plus sea zones and wastelands. I'm not particularly good at map modding, but I have enough experience with it to say that it would be quite a bit of work.
All of the extra counties (and baroines) would naturally mean a lot more characters are running around at any given time and that various triggers (e.g. whatever CK3 uses for any_province) will need to check more things, which would mean performance is affected. How much it would be affected is hard to say (particularly as the devs potentially can do things to improve it that a mod could not), and whether it would be a reasonable trade-off is going to be very subjective; anyone that's either using a computer with the bare minimum specs or that has zero interest in anything further east is most definitely not going to be in favour of any significant performance drop, and just where someone not in either of those categories would draw the line is hard to say.
Secondly, you're going to need to add a lot of characters in the history files. I'd ballpark the number of character (both historical and fictional, and both after 1066 and not) vanilla CK2 added for the areas relevant to JD (Tibet, Nepal, the Tarim Basin, offmap China, and a small section of the steppe) at less than 2k, while I'd say that the historical characters you'd need in 1066 and prior would most likely be north of 5k for something like this if you want to be as thorough as elsewhere (and given that you can find e.g. Roman Emperors back to Augustus (at least in CK2) you probably want to be pretty thorough and add e.g. the Three Kingdoms in China, seeing as the people that care about the area are unlikely to be pleased by a much poorer job), plus various fictional placeholders holding counties in areas where there isn't any historical information (and possibly also added in areas with historical holders/top lieges if those holders end up with an unreasonably large demesne). How many fictional placeholders you'd need depends on both how granular you want to be when it comes to breaking down (sub)realms and whether you want a continuous line from 867 to 1066 (and/or heirs or family members in either start), but even using large (sub)realms and the bare minimum number of characters (one ruler in 867 and one ruler in 1066) you're probably looking at over a hundred characters, and you're both going to want to make sure that you actually need the placeholders and that the culture/religion/name/etc. is reasonable for all of them as people otherwise are likely to get offended.
It would be a lot of work to add those characters (I suppose the CK3 devs could reach out to CK2/3 mods that have added any relevant characters for assistance to reduce the workload to some extent, but they're probably going to want to double-check at least some things and might decide that they need to make changes in some places, and unless it's a CK3 mod with zero character id collisions it'd require more work than just copy-paste), particularly as a lot of the information you can find is incomplete, contradictory, and/or written in Chinese/Japanese/Korean/etc. and thus unlikely to be something the average dev can read without some kind of assistance (be it a a human translator, a dictionary, Google Translate (which doesn't do great with certain languages...), or something else) even if they can find it in the first place, unlike something written in English or at least a language that uses the Latin alphabet. Reading up on reasonably obscure historical people can certainly be interesting, and it can give you ideas for flavour and possibly ideas as to how you could promote the map expansion (and you might even be able to win over some initially sceptical people by showing off some really interesting historical starts), but it would without question be a huge resource investment just to get the historical setup done, and unless a significant chunk has gotten done it isn't all that exciting to show off (meaning there'd either be silence, filler dev diaries, or a need to do other things on the side).
Thirdly, you're going to need to add a bunch of cultures and faiths with a certain amount of flavour (and finding information about all of those faiths isn't necessarily going to be easy), a good number of new mechanics (even ignoring certain things only relevant after 1066, you're going to have to contend with some messy things in Japan, what with the Tenno being a figurehead (but the Imperial Family's possession of the Chrysanthemum Throne being sacrosanct) and the Fujiwara regents calling the shots, and that's just one area), and likely significant adjustments to existing mechanics (for example, the circumstances under which someone creates a new House would probably need to be looked at as you e.g. don't want distant members of the Imperial Family to disinherit themselves by de facto giving themselves a surname).
Even with certain concessions to gameplay (e.g. letting you inherit a minimum of one county even in areas that didn't really do dynastic inheritance) -- which will be contentious, as not everyone has the same idea as to what constitutes a reasonable deviation from historical accuracy -- and a certain degree of simplification when it comes to culture/religion (which sometimes makes people unhappy if something they'd have preferred didn't make the cut), this would be a not insignificant amount of work on top of the previously mentioned historical work, and given that people both are going to expect a certain amount of flavour right away in an official addition (because "We've done the historical setup, but flavour will have to wait until a later release" isn't really going to be acceptable there; speaking from experience, there's grumbling even when mods do that) and are going to get annoyed by an update that takes a long time to get done (speaking from experience, this happens even with mods) it might be prudent to do other things instead of (or before) a Far East expansion (and having a secret Far East expansion worked on in the background while releasing other things doesn't really seem like a great idea, since that's easily going to cause complications).
Also, some things relevant to the area might require functionality that does not yet exist in the game. That additional functionality would likely be appreciated by at least some modders, so it being implemented wouldn't necessarily be bad, but figuring out an implementation (and then actually implementing it) would take a while, and considering past instances of functionality causing issues down the line (which devs that have had the pleasure of dealing with those issues likely would know) or being a pain for modders wanting to tweak/expand on/etc. something (which at least some of the devs would be very well aware of, given that they've got a modding background) it would be prudent to consider more factors than "Does this implementation work for this specific scenario?".
Fourthly, there are some obvious balance concerns. Most notably, China (Tang in 867, Song in 1066) would be a huge blob right at the start (if perhaps not quite as huge a blob as some people might expect; China didn't control all of China proper in either 867 or 1066), and though China certainly should be the kind of neighbour you need to be wary of a Chinese blob that regularly consumes everything nearby would be pretty inaccurate, seeing as their historical track record was pretty mixed, and some other areas would potentially need to be looked at as well. While any decision regarding how to handle balance would be contentious, and while other content also might have balance issues, it might be prudent to hold off on a big map expansion until such a time that the devs have come up with (what they think is) a reasonably balanced approach rather than going "Eh, we'll just add it and patch it if necessary".
Finally, while there is precedent for PDS adding things that were unpopular (and sometimes even expanding on them, e.g. adding some things related to Sunrise Invasion well after its release despite most people not being remotely fond of that DLC existing in the first place) to one degree or another, the idea of adding the Far East (or "China") is -- and has been since a long time back (I think the CK2 forum regularly argued the topic since around RoI's release) -- pretty high on the list of what a lot of people don't want (if not always for the same reasons). There isn't necessarily a consensus regarding what people want to see instead (everyone tends to have their pet issue, and a good number of people are unreasonably attached to those things) or how something else that's desirable should be implemented -- and some other ideas might be even more unappealing to people (e.g. maybe people would rather see the Far East than the return of supernatural stuff) -- but chances are there are plenty of other things that'd be overall more welcome and a good number of smaller things that the devs deem feasible and desirable to do alongside those things, so it is pretty likely that a Far East expansion isn't exactly considered a high priority at present even if it might exist somewhere on an internal "wish list" (or even on an internal roadmap).
TL;DR: It would be a lot of work, there are performance and balance concerns, and it wouldn't necessarily be well received, which means it's probably not something that'll get done in the short term if it gets done at all.
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