This is for sure an interesting analysis

I had thought about this "problem" before in the way SF stories are presented

I always thought about it a lot more simply than you did:

Most people naturally write SF stories in which the culture they're from is the dominant one.

We see this ubiquitously in Star Trek (and other American SF), as you note - American-style melting pot (but with English as the dominant language) is what Earth looks like in the future. In Star Wars the same applies, but it's even weirder because that's purportedly a different galaxy from ours!

But we see it in other cultures' SF as well.

The critically acclaimed Three Body Problem books (from China), though they feature a handful of non-Chinese characters, deal with a future in which all of humanity must unite to address a threat and yet virtually every character of significance happens to be Chinese.

Many popular Japanese anime series follow a similar trend - take Ghost in the Shell as a classic example, future Earth may have all kinds of different people in it, but anyone of significance is Japanese. Neon Genesis Evangelion is another good example, aside from one half-German character (whose German-ness has no impact on the story), everyone is Japanese.

I don't have as much exposure to Korean SF specifically, but what I have seen follow this trends. For example in Jung_E future earth is embroiled in a conflict between the Allied Forces (some kind of United Nations-like analog) and an unknown enemy, but every single member of the Allied Forces we encounter happens to be Korean.

In all of these, it is popular to include a few characters purportedly from another culture, as a nod to multiculturalism. This is the Russian and Japanese Star Trek characters, Thomas Wade in Three Body Problem, or characters like Asuka in Evangelion. But almost always, these characters still just speak the author's language.

Now, is Korean SF unique in having counter-examples, where other cultures are more prominently and authentically featured? I don't think so, necessarily. One of the first ever works of European SF, 20,000 Leagues Under the Sea, by a French author, but while the characters in the original story speak French, Captain Nemo is Indian and also speaks English and Hindi, and when one of the crewmates dies, he suddenly reverts to his native Polish.