I see it like this. Books are the work of a single individual. That one person will have broad authority to write their story however they please. So range of book quality is very large. There are great books and there are truly awful books. And in fact, the vast majority of books are total rubbish. But the dregs get forgotten and the good stuff rises to the top.

Movies are made by committee. This reduces the spread of quality. Many hands tends to move things towards the average. So you have a much lower portion of total crap, but you also don't have as many true masterpieces. The quality of most movies tends to be pretty mid.

But because books don't go through as much of an averaging out of quality through being created by many hands, when they go well. They go WELL. Sometimes a master author will sit down, truly be in their element, and create their greatest work. And their vision will carry through and arrive to the reader undiluted. But movies? You can be the greatest director or screen writer on the planet; you're still not going to be able to make a movie without the help of hundreds of other people. You could write the world's greatest movie, but your vision will inevitably be worn down quite a bit before it reaches the audiences in theaters.

Or, expressed graphically:

Yes. And let's not forget that making a movie is infinitely more complex than writing a book. For a book there's usually a single author. Sure, they might get feedback from editors and friends, but ultimately it's just the author. A movie requires a load of talented people and their artistic vision and abilities need to align. Script, director, photography, editor and so many other departments need to come together to create something good or sometimes even great.