Why didn't a single language predominate China, Japan, Korea?

Question

Because related Indo-European languages in the West spread enormously, why didn't related languages of some family (an obvious possibility is Sinitic) similarly spread through the areas that are now China, Korea, Japan? Why didn't a sole (ancestor) language preponderate over these areas? I assume nothing about what that lone language was, but Chinese feels like the most probable candidate. I'm extending this question on Reddit to Chinese.

Unquestionably China, Korea, Japan are way closer to each other than Iceland, Ireland vs. Bengal — the "polar opposite" language borders of the Indo-European family. China and Korea share land borders. The shortest distance from Korea to Japan is 214 km, from Busan to Hakata.

I'll analogize Chinese, Japanese, Korean to Latin and Latin's devolution into the Romance Languages. Where does my analogy fail? Did the East Asians lack horses and wheeled vehicle technology of the Indo-Europeans? Korea and Japan adopted Chinese, the Chinese writing system, and culture. "The Chinese hanzi is in fact Korean hanja and Japanese kanji". I know about Sino-Korean and Sino-Japanese words, but Modern Japanese utilizes Chinese characters more explicitly and readily than Modern Korean. Similarly, Western Europe adopted Latin, the Latin writing system, and Roman culture.

Like the Roman Empire, China has been a military, economic, and social superpower that influenced Korea, Japan, Vietnam. "Korean and Japanese officials and intellectuals used Chinese for the official documentation, records and poems, just like British used [Norman] French in their courts for some time before their native “English” language could be widely accepted and used". I'm surmising that Chinese could've been a sprachbund and lingua franca.

Whether Koreanic and Japonic languages share a common origin is an outstanding question — but this is irrelevant — because even if they are related, the Sino-Tibetan language family is distinguished and distinct from the Koreanic/Japonic language families.

I read that the Japanese migrated not from mainland China, and "the Manchurians (related more the the Mongols and Turks than the Han Chinese) migrated to Korea, and then the Koreans crossed the straight into Japan".

Answer 1

The OPs comments clarify an erroneous assumption – "why didn't Chinese cause Korean and Japanese to become more related to Chinese than they are currently?" and "I was asking why Korean and Japanese didn't borrow more from, or liken themselves more to, Chinese so that Modern Korean and Japanese can enter the same language family as Chinese".

In the genetic concept of language family, languages are not "adopted" into a language family. In this view of language relationships, Tamil is a member of the Dravidian language family and is not a member of the Indo-European family, despite the huge number of Sanskrit loan words into Tamil. Swahili is a Bantu language and always has been on, regardless of the influence of Arabic.

Some languages do not have a single ancestor, in particular pidgins and their offspring creoles, where the former arises by groups of adults needing to communicate with no common language, and they concoct a limited language. In that situation, we would not say that the resulting language is a "member of a family", but it could be the start of a new language family.

The OP also suggests that "language spread" could somehow be relevant to there being a single language family. This may be true, in the sense that when one language spreads, it may obliterate indigenous languages. The expansion of English, Spanish and Portuguese in the New World are a prime case of that happening. The explanation for why language A did not obliterate languages B or C is not linguistic, it is social and historical. The continental Celtic languages were obliterated as a result of the expansion of other tribes (Romans, primarily). The Germanic language English was not obliterated as a result of the Norman conquest: one should look for social and historical explanations for the differences.

Answer 2

Because there were neolithic agricultural speakers of extinct language families beside PST people, their technologies were even more advanced. Look at Hongshan, Dawenkou and Liangzhu cultures. Some families might survive until today, probably including Hmong-Mien, Kra-Dai and Koreanic.

About Dongyi (Eastern Yi peoples): There were technically advanced peoples occupying coastal eastern China, especially in Shandong and Jiangsu. I've seen some basic words which have problems to be traced back to PST, but have correspondence with Middle or Old Korean:

风（風）*prəm "wind" vs MK polom,

江 *kˤroŋ "river" vs MK kolom,

木 *C.mˤok "tree" vs OK namok

Some words above are also present in Austroasiatic and/or Austro-Tai, but they couldn't affect Proto-Koreanic so much into basic vocabulary. I personally suspect these words were originally loans from extinct Dongyi languages, being borrowed into adjacent families.