Why are the movies usually worse than the books, because a book only has one author and that author is hardly under any time pressure to write his/her work until he/she finds it perfect.
Time often plays a factor in why movies don't hold up to books.
Movies are constrained to 90-120 minutes to tell the story. Which doesn't give as much time to adequetely convey things that are in a book which is often critical for the overall story (Even more so when a single film often spans multiple books in a series). You lose a lot of build up and worldbuilding when things are rushed through so that an audience can reach the end within the couple of hours the film lasts for.
This is also why video game movies often are terrible too, since even though they both share the multiple writers, the need to cram a 20-40 hour game's story, worldbuilding and drama into a fraction of that usually ends up with something that ends up being shallow and uninteresting, with only action scenes being notable because nothing else has had the exposition to flourish.
On the flip side, when a media is simply used for the setting rather than trying to redo a story in film, results are better. The FFVII film Advent Children simply uses the setting and characters from the game and the film thus focuses on having a story written for the film specifically, so its pacing is appropriate for what is being told and as a result the film is actually good.
Such things can arise in other forms of cross media products - Such as books or films turned into video games are almost universally garbage. Since writing needs to be done for the specific media or it tends to fail (With things turned into video games also often having the sin of removing one of the best parts of video game storytelling - Player interaction. Since if a game railroads a player into the strict linear path of the book/film, you lose the reason to even have it be a game)
