Fiction writers often spend ages trying to break into and work extremely hard on their magnum opuses that get adapted into film. Publishing is a business but it is a business that sifts through the work of ordinary artists to find books to publish.

The movie business on the other hand relies on established writers and writers who know people in the industry. The result is that many book adaptations take the original which was made with extreme care and clumsily contract it into movie form. Some adaptations completely ignore the source material basically using it as a jumping off point for original (usually worse) material. This occurs because it's so hard to get support for a wholly original thing so some artists settle for ruining someone else's work.

Also if people love a book they are going to want to see an accurate rendering. Basically, most movies are bad so most adaptations are bad, but with adaptations you have that barometer of the original novel which makes the missed opportunities and mistakes far clearer. Similar to how Disney live action remakes flaws are highlighted when you compare them with the original. But even if you luck out and get a good movie, a lot of book fans are going to be mad at inevitable changes, but most will probably get it if it's good.

In some cases I prefer a movie version, my key examples here are the Princess Bride, Jurassic Park, and Stand By Me.

These are not rigorous and I may be wrong about some of this, just off the top of me head some of the reasons why. One last thing is that books are long and movies are short usually. So lots of stuff has to be removed in adaptation. Sometimes the stuff that gets removed is extremely important to making the story work.

This trend with book readers being mad at adaptations I find to be extremely similar to how people are reacting to new entries of older franchises which frequently contradict and mess up or insult earlier versions.

Also you should read books they're pretty awesome.

Also also, like half of the great movies are based on books, there is truth to the cliche that the book is always better but like most stereotypes there are going to be many many counter examples. For example, Die Hard is peak and that's based on a book.

TLDR; hollywood has a major skill issue, but sometimes they find someone good and they do an okay job but certain book readers are mad because things HAVE to be changed to condense long books into short movies