FWIW according to wikipedia there are 9.9 million Kims and 2.9 million Smiths. Kim is the most popular Korean surname and Smith is the most popular English surname.
I believe that I have yet to meet a Korean whose last name is something other than Kim, Lee, or Park.
[edit] I was wrong I know a Cheong as well.
Kim Lee and Park only make up half the population. You may just have falsely categorized Koreans with other last names.