The vocabulary that defines geographical terms was always politically loaded and never meant to be interpreted in a scientific, rational manner.
South east asia doesn't literally mean "everything that is south and east in Asia". Just like how Europe isn't an actual continent but an arbitrary border that defines itself, in reality, by "the place where white people live in the West of the continent and who aren't Russians". Because the actual contiguous landmass, is, well, Eurasia, and Europe is an arbitrary human color that paints something that doesn't exist over the world.
https://dppa.un.org/en/northeast-asia
The United Nations define Northeast Asia as being: Japan, China, Mongolia, South and North Korea.
From wikipedia :
> The term Northeast Asia was popularized during the 1930s by American historian and political scientist Robert Kerner. Under Kerner's definition, "Northeast Asia" includes the Japanese Archipelago, the Korean Peninsula, the Mongolian Plateau, the Northeast China Plain, and the mountainous regions of the Russian Far East, stretching from the Lena River in the west to the Pacific Ocean in the east
>The definition of Northeast Asia is not static but often changes according to the context in which it is discussed.
If you think really hard about what those countries have in common, something that isn't related to a rational view of geography, it's that they're asians who don't look too brown. Yes, the origins of the definitions of SEA vs NEA are racially motivated and they're terms we're stuck with because most of the world knows what you mean when you talk about SEA vs NEA.
https://src-h.slav.hokudai.ac.jp/publictn/acta/15/kotokin/ko...
> "On the intimate frontiers between the Russian and the Chinese empires," Kerner wrote in the 1920s, "are face to face the world's largest, fastest-growing white population and the world's largest, ablest and perhaps fastest-growing yellow population." Much the same could be said seventy years later, even if we would disavow the racial categories that provided much of the attraction for Kerner
Those definitions were written by people who didn't shy away from calling others "Yellow population". But they also became part of the language enough that even entities like the United Nations won't bother trying to change them.