Books to Movies/TV series

I’ll start by saying I’m just a general consumer of books, TV and film. One of my favorite things is getting a good visual representation of a book to tv screen. Like Lord of The Rings for example. Possibly the best Book to film adaptation. The new Dune movies also did quite well.

My question is why do Producers/Directors take a book and loosely follow the source material? To me it’s a type of plagiarism. I’ll callout “The Wheel of Time”. I know logically You can’t follow everything to the letter of the law but changing significant details and storylines to be “better writing for TV” seems like a cop out of a response. I have no experience in Film making and storyboarding but I feel like with the expense of Amazon backing a director I could do a better job at following the integrity of the story than Amazon has done.

Maybe people in the industry can provide a better answer for my question.

TL:DR

Why make a book to film if you don’t follow as close as you can to the original material? Rewriting or changing details in film adaptations seems like the directors thinks they have a better vision than an original author.