I get that this is a laugh-at-the-white-guy post, but it’s actually an interesting question.
In the US, the 100 most common surnames represent about 18% of the population. In South Korea, the 5 most common surnames represent more than 50% of the population, with Kim/Gim/Ghim/김 alone being around 22%.
The answer, best as I can find it, is that surnames originally in Korea were much more recently brought to the average person.
In Anglo culture, average people having last names dates back to around the Middle Ages, and your surname could be anything from your profession (Smith, Taylor, Cooper, Chandler, Fisher, etc.) to your geographic location or some feature of it (Barrow, Liscombe, Badgerly, Wyndham, etc.) to some defining feature of you or your ancestor (Red, Brown, Small, Little, Longfellow, etc.). These were reasonably widespread in use and are the origins of most Anglo surnames today.
In Korea, until relatively recently, surnames were only used by the nobility and aristocracy. The ruling family for a long time in some rich and important regions? 김. And so, when everyone started taking on surnames, it was only natural to have your surname (which, remember, was largely geographic in nature, although not named after a geographic location per se) be the same as your ruler. And so you end up with more than half the population sharing the same 5 last names.
A similar thing is at work in China with the surname Wang/Wong/王/汪. The two variations are most common and 58th respectively. The first of these, by the way, 王, means King, just to make the connection a little more obvious. It also represents more than 7% of the population all on its own.
Loki is the Philosopher King 