If I had to guess, I suppose the comic industry in Korea wasn't as established when webcomics started to take off. In the USA, most of Western Europe and Japan the issue/volume/magazine format is still somewhat the norm. A fair amount of webcomics is still based upon this format (though not many Koreans I'm aware of). Combining this with what AbyssalMonkey said, I can see something resembling an answer.
Going an a tangent, this is an interesting post about webtoons : https://hoxtranslations.blogspot.com/2014/08/some-thoughts-about-webtoons-and.html