Why are East Asian Countries so Ethnically Homogeneous?

Why are the countries of East Asia so seemingly ethnically homogeneous? Most countries of East Asia, including China, Korea, Japan, Taiwan and Mongolia have a far more homogeneous or uniform national identity when it comes to a cultural, ethnic and linguistic perspective when compared to other countries of Asia, or even Europe, but why is this the case? Today we will be looking at the genetic and ethnic history of the region stretching back to antiquity, and how and why many of these East Asian groups came to be in the first place, and what happened to all of the ethnic diversity that may or may not have existed in the first place. Thanks for watching!