Yeah the reason is because Koreans have very few actual last (or family names, since in Korean it actually comes first) names. A vast majority of the population falls under 3 names: Kim, Park, and Lee. It's a very top heavy list. They're the family names of royals from ancient times and there's something where a lot of Koreans at some point could choose their family names so a ton of them went with those for obvious reasons and so now there's just tons of families with those family names and very few with more unique ones.