I guess I've read enough xianxia that the idea of the human form being the ultimate form doesn't surprise me even in manga anymore. Maybe it actually is an East Asian thing in general, I wouldn't know. Though that being said, even in the Bible it's written God created the human after his own image, so it's not like the idea wouldn't exist beyond East Asia. From a purely writing technique point of view, a more or less humanoid monsters is easily made more menacing than a random, beastly monster because humans typically only associate deeper emotions and intelligence with other humans. If a beast kills a human, it's just following its instincts and that sort of things, nothing personal, but if a human randomly kills another human, it's an act of evil intentions.