Why does he not give ratings to jazz albums or (more importantly) classical albums?
Because that requires him to actually know music that academics could call him out on. Rock is younger and has fewer academics and so he can get away with with more.
Besides, do you really want him to go around saying Weather Report is a 3/10 at best?
there's already plenty of worthwhile jazz/classical criticism and scaruffi's taste in those idioms, based on his lists, doesn't really stray from the norm. by contrast, rock has a dearth of worthwhile criticism, and scaruffi's taste is far more iconoclastic. it makes sense for him to focus more of his attention on that.