Since when has Korea and Japan represented Asia?

No question Korea and Japan have a fabulous culture and cities and friendly, nice people. I don't mind encouraging anyone who wishes to visit those countries to spend their money for it. But I don't really understand why people outside of Asia kinda have this notion that these two countries are representations of Asia. What do I mean? Ask anyone what they know about Asia they'd talk about kimchi, sushi, ramen, those korean dramas, some kpop or jpop band, anime, school girl uniforms, naive cutesy demeanor, etc. Asia is too big and diverse to be encapsulated by two countries. A lot obviously have no idea how South East, West, and North Asia and other East Asian countries are just as interesting as Korea and Japan in their own ways. I find it quite insulting for other Asian nationalities because it's tantamount to pretending like they do not exist. Other than Korea and Japan, I suppose the rest of Asia deserve some little appreciation too or if not at least some respect by being equally recognized as facets of Asia as well and not being placed in Korea and Japan's shadow.