“Don’t judge a book by its cover.” Everyone knows that, right?
But the entire reason that books have covers is so that you can judge them! In a bookstore, nobody’s going to take the time to read the first chapter of every single book! They’re going to look at the cover. Judge the book by its cover. And if the cover interests them, they might buy the book. If the cover is not interesting, nobody pays enough attention to the book to buy it. Simple as that
So, why do people say “Don’t judge a book by its cover?” Why not use the Wikipedian alternative, “you shouldn’t prejudge the worth or value of something by its outward appearance alone”