This is their current situation, but historically this has not been the case. Korean for a very long time was written with Chinese characters, as was Vietnamese

This is their current situation, but historically this has not been the case. Korean for a very long time was written with Chinese characters, as was Vietnamese. The article posted by GP also clarifies a distinction between adopting Chinese loan words as any given language may (such as the word "tea" in English), versus a particular model of bulk adoption of not only characters but vocabulary that occurred in JKV languages.