I don't know about writers of all puranas or vedas but when ever I hear about a writer of any purana or veda then it's always a human. Are only humans allowed to write puranas and vedas? Is this a rule, a coincidence or anything else?
There are many more yonies like Deva yoni, Yaksha, Naag, Rakshas (there were some great rakshas also) and many more whose name I don't remember right now. Why they(people of any other yonies or lokas) didn't write any Vedas/Puranas? Does that mean, devotion can be found in humans the most? Or there is another reason? Or may be they have written but I am not aware of it.
The only non-human example I know is Ravana. I heard he is the one who write all vedas in form of music or song/poem, so that people can (kind of) sing shlokas and that will be helpful for people to easily grab knowledge because when we learn something in musical form then we understand much easily and early.
But Ravana did not write all vedas by himself. Vedas already were written, he only translate them in musical form on request of Lord Shiva (that's what I heard).