I think it took me a while to understand (after being told by native speakers) that Korean and Japanese are a bit of a linguistic evolutionary puzzle, especially if you thought that because their characters look like Chinese, their spoken languages are related to Chinese. They are not. The languages must have come first / evolved independent of China, and then a wave of foreign commerce / scholarly / government influ…
One difference is that Rome fell and the Normans brought a highly changed Romance language with them into England. While Korea and China had an ongoing relationship going back millennia. As Chinese changed over time different words from different eras also crept in to the language.
On top of that, Korean itself also changed, harmonized dialects, dropped and added sounds and features and other things. The Korean written in the first book on Hangul by King Sejong the Great is both not only not pronounceable by modern Koreans, but less understandable than Chaucer might be to modern English speakers.