Korea, and its cities in particular, have very high population densities, as you might know. To gain high returns on such a limited amount of land, development companies started to build higher houses instead of larger ones. Each new apartment block is higher and more unique than the last.
People here are competitive and everyone wants to live in a higher apartment. The higher the apartment, the more expensive it is. As a result, Korean cities have become filled with high-rise building forests and all the cities have become so similar to each other.