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People could either buy themselves into a genealogy or do it through forgery, forgoing their own surnames. There were advantages to having a ‘noble’ surname:

• thaumasiotes 3y ago
  Note that a surname in the top eight is, by definition, not a noble surname. Almost everyone bearing such a name is a commoner and having it is nothing you can brag about. Your link suggests that what was for sale was noble identity - a place in a specific and unique noble family, with formal documentation that you belonged to it - not a license to use the surname, which everyone was already free to use. > It became increasingly common for successful merchants too to take on a last name. They could purchase an elite genealogy by physically buying a genealogical book (jokbo) — perhaps that of a bankrupt yangban [noble] — and using his surname. > By the late 18th century, forgery of such records was rampant. Many families fiddled with theirs: when, for example, a bloodline came to an end, a non-relative could be written into a genealogical book in return for payment. The stranger, in turn, acquired a noble surname. But again, what you're acquiring there isn't a "noble surname". It's noble legal status, and you end up with whatever surname the noble family uses by coincidence. No one participating in this system cared whether they ended up with the name 金 or with 網切, so this isn't a good explanation for why 金 is so common. The link also dwells on the much greater variety visible in Japanese surnames. It doesn't point out why that is, but we do know the answer - they are more variable for the extremely obvious reason that the concept of a surname in Japan is much more recent, dating to the late nineteenth century.