The reason why that is is books are free to take as much time as they want in telling their story, but a movie typically has to be less than 3 hours long or if it's a TV show everything has to be condensed into hour long episodes with a beginning, middle and end, which isn't quite how book chapters usually work.
So you know, often times when a work is condensed you lose a lot of detail and overall flavor that can make a work shine, if you're familiar with the book oftentimes the movie can just feel like nothing more than a cliff notes of the book.
Jurassic Park, the book and the movie is a fascinating case study of how to adapt a book and the pros and cons of movies vs books.
Jurassic Park was obviously a story just BEGGING for visualization and that alone give it a huge advantage over the book, but the book has a lot more going on, a much deeper story and can get away with something the movie couldn't, namely gore, which balances out the lack of visualization by making the dinos in some ways even more intimidating in the book (the Nedry death scene, book vs movie, is a perfect example, the movie plays it for comedy, the book plays it for horror,)
Basically imagine if the JP movie actually showed dinos munching on people's guts rather than just implying it.
But Jurassic Park was very, very cleverly adapted into a film, while the very broad outline of the story is the same, the Jurassic Park movie was more of an adaption of the "premise" than it really was an adaption of the book itself, which frees up the movie to be more of it's own thing and I think that's the best way to do it, adapt the premise and the broad outlines of the story, not just a condensed, abbreviated version of the book.
So consequently both Jurassic Park the book and the movie are equally worth experiencing, neither is really better than the other, they're just two good takes on the same basic idea.
However it's a tricky challenge to make something that can stand toe to toe with the original book, for another Spielberg example you have Ready Player One, which the RPO movie removed almost everything that was interesting about the book (and it wasn't a GREAT book to begin with) resulting in a movie that to me was just an inferior version.
RPO was a story that actually favored being a book over a movie, because you don't have to worry as much about copyright like the movie did and you can get into a lot more nitty gritty detail of the world and how it functioned.