My comment may deserve a downvote, but great question and asked in the right forum. Asking this question from 100 randomly picked Japanese people is probably as likely to produce the right answer as asking 100 randomly picked Americans why "he don't" is incorrect, but here someone may actually know. Just speculating, but maybe rivers where "discovered" and named later than lakes and mountains, since while one can see (the entire) mountain and lake, you can only see the very local part of a long winding river) unless you spend extra efforts. — Tuomo 11 hours ago