I disagree. It's not about a superiority of art form in general (because you are right they have different strengths), It is about superiority of writing. That's why writing from the book can easily be simplified to fit the form of the movie, but it's hard to do it the other way, because without audiovisual effects reader quickly realize that's it's plot is dumb/simple.Um, no? First of all, there are quite good books on the movies. Read Star Wars Episode III: Revenge of the Sith by Matthew Stover. Secondly, the problem with game/movie novelizations and book/game adaptations is not the superiority of any one art form,
Take imdb top 10, six of these movies (Godfather I and II, Schindler's list, The Shawshank Redemption, 12 Angry Men and One Flew Over the Cuckoo's Nest) are based on the books, so it's a majority of them. Of course their level of fidelity differs, but still the creators had a base on which they were building. The fact that some book writers are also pretty successful as screenwriters also confirms in my eyes that writing for movies is just easier.
As for games - interactivity is indeed their strong side, but it often enforces simplification of the story, so writing is generally inferior to the movies