Why do the plot scenes and characters often differ between books and movies?

A book is a medium where imagination reigns. The book author must describe the situations, events, and characters to guide the reader's imagination through the path which the author wishes the reader to pass. The book author must describe, with words alone, not only the main focus, but also any relevant surroundings, while leaving less important facts to the reader's imagination. A movie is a medium where the director's imagination reigns. The movie director, as well as script writer and others, is forced to show, from whatever point of view she chooses, reality fully realized. A scene must be fully realized through the use of sets, camera angles, and character positioning. More limiting still, the scene must be realistic, yet vivid, in order to retain the interest of the movie viewer. Some things that work well in the written word alone just don't work on the big screen. It could be in the time it would take to show the scene, or it could be in the cost involved in bringing to life a particular plot element more easily described by the written word. Someone once said that a picture is worth a thousand words, but if the picture cannot be realized realistically, few of those words are worth anything.