I hear that all the time when people talk about movies they've seen. If I may go on a bit of a rant, this is one of the most annoying 'critiques' of movies there is, for a few reasons. And it's funny, you don't hear people say, oh the play was better, or the short story was better, or the real life story was better. Just that the book was better. There's a meme about how you can tell if someone's a vegan, and I think there could be one for people who have read the book. They love spewing their expertise since they read all 200, or 400, or 700 pages over the course of a month why it's a travesty that a film was unable to disseminate all the information that was in the book in less than 3 hours. Which is why that's a ridiculous criticism of movies.
Movies are short (in comparison). You can read a script of a film in about two hours, or generally as long as the movie will end up being. I don't know if I've ever read a novel in that amount of time, even a short one. So when a movie is being adapted from a novel, certain choices have to be made. Some of the digressive side stories have to be expunged in a process many creative people call "Killing Your Darlings".
Second, movies are visual, whereas novels are not. Novels require lots of description to set a scene, and God help you if you can't tell the characters apart by their voices. Movies have an advantage here, in that you don't have to use your imagination, because someone (or many someones) have already done that for you. So, in a novel, authors have to distinguish between characters and set scenes by adding in a whole lot of inner monologue. A lot of digression happens here, too, as characters go on long random tangent about their feelings, or about how this reminds them of another thing that happened when they were young (which they will of course tell you about), or they'll wonder about other things and people (often as a foreshadowing device). But this is always happening in their head.
Which doesn't work in films.
There is a commonly accepted belief that any narration by a character should be kept to a minimum, because, big surprise, it slows the narrative down, and we have a limited amount of time. Just like Smokey, we've got a long way to go and a short time to get there, so we can't stop to examine your precious character's feelings. Or, as they say in the biz-
SHOW, don't tell.
Don't get me wrong, I'm and advocate of reading. Not enough people read past reedit or facebook posts. Novels are great. So are short stories. So are plays. I'm sure poetry has its fun times too. I love books. I have more of them than I should, and yes, I have read many books that have been made into films. And yes, I have wished for those films to be better, but not because the book was, but because the movie should have just been better than it was. Like Forrest Gump. Or Water for Elephants. Sometimes I even avoid reading the book (if I know it's going to be a movie) until after I've seen the movie. There's nothing worse than knowing what's coming because you were the one that spoiled it.
The other reason folks should refrain from comparing books to movies (and I know it can be difficult), is because they are two different art forms. Say I was inspired to write a song because of a painting I saw. Would you compare the two? Would you compare them even if they had the same title? No, you wouldn't. Because they are not the same thing. They are forms of art that have different tools. Novel has words. Movies have words. But movies also have pictures, music, acting, fashion design, and painting. Novels might have those things in your head (and yes, sometimes you imagine them wrong), but they don't really exist outside your noggin. It's only words. Just like this blog. And I can guarantee, that if for some ridiculous reason this blog is ever made into a movie, some doofus will go around saying the blog was better than the movie.
Some movies I like that were based on books:
Of Love and Other Demons
The Hunt for Red October
In the Name of the Father
Willy Wonka and the Chocolate Factory
Silence of the Lambs
Stand By Me
The Last of the Mohicans
The Princess Bride
Harry Potter (take your pick)