“The book is always better than the movie.” This presumption is widespread, but it is less a critical determination than a personal bias. A movie based on a literary source is often seen as a secondary work and, consequently, of secondary value. Literature, generally, still occupies a more privileged position in the cultural hierarchy than movies do; and readers often have a proprietary attitude toward the book, an attitude that influences their reception of a film based upon it.
Cahir, Linda Costanzo. Literature into Film : Theory and Practical Approaches, McFarland & Company, Incorporated Publishers, 2006.

Movie-making is big business. It is expensive, arguably the most expensive art form. In contrast, literature is, arguably, the least expensive art form. All that is needed to create a literary work is a pen and paper. Literature can be, and normally is, produced by a single writer, who needs to secure no approval outside of his or her own. Film is produced in a very di›erent way.
Cahir, Linda Costanzo. Literature into Film : Theory and Practical Approaches, McFarland & Company, Incorporated Publishers, 2006.