There will always be people who would rather invest the time to read the book rather than watch the movie. If they even go see the movie, they will often be heard to comment that the book was better. This is not always true, and there are some good examples of books that turned out much better in movie format. These are generally books with uncomplicated plots that lend themselves to great special effects when shown on the big screen.
The purpose of entertainment is to take people out of their everyday lives, and show them a different or exciting view of the world. Movies do this by telling a story visually as well as with sound and acting. They have approximately two hours or less to set the stage, manage the action and conclude. Books have hundreds, sometimes thousands, of pages to tell the same story. It is little wonderful that dedicated readers prefer books, but movie goers want their entertainment encapsulated.
Producers and directors are always looking for great stories to tell. They cannot make a movie without a script, and books have often been the basis of modern movie scripts. Scenes, characters and details within the movie are different from books because of time constraints and dramatic license. This does not detract from either form of entertainment, and it gives both forms their own way to please a specific audience.
Books are not the basis of every movie, and they should not be. There are some books that can only entertain readers because they are too complex to be reduced into a movie format. For some of them, this might change in the future with new technology. Other books might possibly be made into niche movies, but these must wait for the limited funding available to independent film makers.