“The book is always better” is one of those pieces of conventional wisdom that people tend to parrot back and forth to each other without really believing. It’s obviously untrue in a dozen different ways: even if we leave aside all the ways that books and films being just fundamentally different artforms makes direct comparison reductive at best, I don’t think anyone would argue that The Godfather or Jaws are better books than films, because the books are enjoyable pulpy novels and the films are masterpieces. Besides, good and great films are adapted from books that nobody cares about, or has even heard of, all the time. It’s hardly worth taking “the book is always better” seriously as an idea because the weight of counterexample is so strong.
But people still say it, as a way to fill a silence if nothing else. You mention some new film adaptation of a literary classic or a recent bestseller, and they say, “well, I think the book is always better,” as you nod along sagely, even though neither of you actually think that Alfred Hitchcock’s Psycho is a pale imitation of the novel. “The book is always better” is just the visible trace of something larger, in how we think about adaptation and how different mediums relate to one another.