For one thing, movies have a time limit. Thus the movie can only show the most important parts of the plot.
Also, movies are what the director and the producer(s) envisioned.
The movies are not always faithful to the book. Who has seen The Hitch Hikers Guide to the Galaxy? In the movie, Zaphod Beeblebrox has two heads, one on top of the other. This is wrong. The book made it ultra clear his heads were beside each other, and thus they occasionally disagreed. In the movie, his lower head was more of a neck, an add on. And many of the adventures in the stolen spacecraft never made it to the big screen. Also the robot Marvin was not how I envisioned him. There was also a whole host of other minor characters that did not get any movie time.
Green screens and computer graphics have come a long way, but they still reflect the directors & producers vision. In the movie 2001, Stanley Kubrick consulted with Marvin Minsky, a computer guru of the time. Minsky convinced Kubrick that HAL should be represented by a single item, not a wall of blinking lights. But even so, the audience is being told how to imagine HAL.
In a book, people can always go back a few pages to confirm the events in the plot.
These are some of the reasons books win. Also, reading a book takes some mental effort. It is not served up for the audience to consume.