To give some insight, /u/TotallyTugBoat provided a great comment

I'm going to upvote you, because I have heard from many Koreans in my area that Koreans are actually racist, and I'll explain why. It really helped teach me not to be so quick to judge. The person I spoke with was a Korean woman who married a white man and moved with him to the US after living much of her childhood and adult life in Seoul. When arriving in the US, she started a non-profit that essentially is an exchange program for college students. Korean college and high school students spent time in the US for a few months, and the same thing happened with American students. The groups would go on historical tours, do community service, and spend a lot of time with other Youth from the country they were visiting. She had a book that she showed me filled with mixed race Koreans living in the country side with the few family members (if they had any) who accepted them (usually it was the mother and no one else, or a grandmother). The book spoke of the difficulty that these people faced even finding jobs. When I say mixed race, they would even not accept Koreans who were half Japanese and half Korean, or half Chinese and half Korean. It was incredible how broad their prejudices were. She explained that without their Xenophobia in the past, Korea likely would not exist. They were constantly being invaded. Constantly, throughout their history. There was a legitimate fear that Korea would cease to exist if they adopted the language and religions of neighboring countries, or if they intermarried. They wanted to keep Korea alive, and one of their defense mechanisms was extreme Xenophobia, even towards other nearby Asian countries. This Xenophobia has resulted in quite a lot of senseless and excessive racist practices in the present day. It's troubling, and there are Koreans working to address these social issues, but when you think about the historical context, it makes sense, and it's difficult to judge. Hopefully with time the problem will improve. TL;DR - Koreans are racist because without their past Xenophobia, Korea likely would not exist as it does today.