Doesn't it explain the reasons if some book or website says so? There are a couple of common features that Altaic, Korean, and Japanese languages share, such as: -SOV -agglutinative -vowel harmony -a word rarely starts with R sound, if not never -a word rarely starts with double consonant, if not never As far as I know, however, many researchers think Japanese is not completely included in Altai family. More researchers think that Japanese is some combination of Altai, Austro-Asian, and some other languages.