AFAIK the historical reasons are well-placed. Japanese Kanji is essentially a spin-off system, since altered, wherein many of the characters that exist in both

AFAIK the historical reasons are well-placed. Japanese Kanji is essentially a spin-off system, since altered, wherein many of the characters that exist in both Chinese (eg. in the Tang Dynasty) and Japanese now have completely different modern meanings. In addition, grammars in both places have since changed (Japanese possibly never really had one, deferring to Middle Chinese in early use, and only slowly adopting characters elsewhere, interspersing other writing systems to provide grammatical structure). Therefore it makes little sense to conflate the two when discussing language, though the evolution of Japanese character use can provide some insight in to Middle Chinese and earlier periods, particularly in subjects such as Nara Period and Silk Road Buddhism. (I am less knowledgeable about Japanese than Chinese, and claim no particular expertise in either, though I do translate old Chinese texts to English for fun and have visited Japan)