Simply put: movies are movies and books are books.
They have to function differently. You don't have time in a movie for a long drawn out character introduction or hints of studying someone before you talk to them.
Interestingly, we have history on our side. Look at the first two Harry Potter movies. Then look at 3 and on when they switched directors. Do you know why they were changed? The movies weren't making money, drawing in new fans, holding the general public's attention. This, as a movie, is bad. While the first two movies were more closely tied to the books in terms of structure, characters, and overall movement of plot and creative things from the books, it largely was apparently too fanciful, hard to produce, and hard to work with with the kids. (Namely movie1, they had to have seperate cameras on each child actor to get "the shot" which is why that movie has a lot of close-up cuts, especially when they were supposed to be in a group talking. The kids just flat out, be silly.) Outside of a budget standpoint, looking strictly at attention holding - that's what movies are designed to do. It's hard to hold a child's attention for 30minutes, much less "feature length" time. This is also I suspect why movies have specific scores, booms, and sounds, and overall are loud messes - the final act of AVATAR is a prime example. The entire movie outside of that arc is overall good, fine, and well paced for story telling, and then the ending is shoved in there like "right we have to actually end this?! AAAAAAAA MAKE NOISE AND OH YEAH THAT GENERAL CAN BE EVIL LETS MAKE IT GUNDAMMMMM"
Summary
Oddly, another example is the The Angry Birds movie. I mean, it's a game based on.... a no-plot app game, so there's not much to go on, but a large portion of the children's movie is based on the character just having pratfalls with no build-up, lead, or pay-off. From what I understand specifically with Fowl's series of books, is that all of the books have notes in the strange characters and the reader will learn to translate them as the books go on - so you can re-read books from the earlier parts of the series and see things literally you couldn't understand before that make that book stronger for it. You cannot "hide" stuff like that in a movie well. Heck, even video games struggle with this.
Another would be, the source material is just... too foreign. Take for example the first Percy Jackson movie. If you've read the first book, you'll know there's some super cool things that happen in that book that do not happen in the movie- and that the movie takes from events that happen in book 5. Why? Simply put, when the script was being designed and put together for the movie, and they were getting information from Riordan, it likely was better to finish a movie (that might be a one hit wonder, like the Golden Compass was) with a large action scene - like allllll movies do - so they decided to slice and cut out the final battle scene from book 5 and paste it into movie 1's plotline, and tweak things so "it could work". Would the scene at the waterpark from book 1 be better and funnier, or them talking to Aphrodite or Aries be a better delivery from the book? Yes. But would the general audience who hadn't read the books, be able to keep up? No. If they see Hades, they have a certain look in mind for him - and Hades in the books is nothing like movie of course either - neither is the afterlife. In fact, the entire opening of the movie is them being like "let's make it obvious these are gods and make them 20ft tall! yeah that'll let the audience know they're important" because it'll remind them of those two movies with Perseus basically saying "dun wanna dad" and "crap" doing it anyway.
In short, movies have to function a different way and the overall goal is to apply to as many people as possible. Why does no one talk about the movie Sinbad? It has the same flare and humor as The Road to El Dorado, and yet it's largely unknown at all. Was it just bad PR or are the paying public picking their movies differently. [There's an interesting video by Film Theorists on yt basically about how movie theatres as a whole likely will be forced to change / shut down due to not making any revenue from the virus. And that one of the highest watched movies in recent months was the Trolls sequel movie.]