There are a few ways to consider this question.
First, we should note that in fact a large number of books are being constantly converted into screenplays. Look at the Oscars and you'll see lots of movies adapted from books. Studios are always looking for new material to which they can secure the rights for a film adaptation. "Branding" is important, and anything with a built-in brand -- like novels -- has an instant edge.
Of course, the fact that large numbers of films are based on or inspired by novels doesn't mean the resulting film adaptations are any good. There are certainly many adaptations that are only so-so or mediocre or just downright awful. However, there are also a great many that are good and even wonderful.
Here is just a sample of lists to demonstrate how many high quality adaptations there are in cinema:
Many writers think adapting a book is great, because it's like having a huge treatment from which you have to just pull out the main story elements and characters and turn it into a visual description on the screenplay page. Others think it's a great process because although the screenplay must be written on its own for a different medium than literature, the book is a guide and source of inspiration, and comes with a pre-existing fan base.
But other folks do find the process of adapting books to be troublesome, even tiresome, and sometimes utterly impossible. Depends on the writer, depends on the book, and depends on everything around and in between.
I've written screenplays and teleplays adapting novels, and it is admittedly hard to distill the essence of a story from a world in which it exists as words and impressions guiding the pure imagination of the audience, and a visual medium that is defined by what you see and hear as well as what you don't see or hear. You have to be able to find the narrative and the arcs, and lift them out of the book in one piece through interpretation into precise visual and audio descriptions.
Words in a book are the final form of the story they are telling, as far as artistic creation. Your mind uses those words to form the images and impressions in your head. The novel won't change from its written form.
But a screenplay is NOT the final version of the story it tells. In a screenplay, there is a very odd situation of the words needing to ultimately cease existing in order for the story to take its final form. The storytelling won't use words on pages, it has to leave behind its former form if it is going to reach fulfillment.
The movie Adaptation is a great multi-layered examination of all of these truths and about the writer needing to adapt -- adapt personally, adapt how they approach and present their material, and adapt the story from the novel into the format of the film. It's one of the best adaptations of a book (it's a non-fiction book about flowers, and Charlie Kaufman was hired to adapt it to film, so the movie he made is in fact about him not being able to adapt a book about flowers into a film, until he gets an ingenuous idea for how to make it work...
I think one thing that makes the process harder is if you make the mistake of looking at it as competing mediums and not entering the project with a mind toward the role of adaptation. The novel's story will have to adapt in order to survive the metamorphosis from the previous medium to the new one. The role of the screenwriter is to guide that adaptation, to retell that story as if they are writing it for the first time, using the story outline and characters from the novel. Remain true to the source, but adapt to transform and survive.More questions on Screenwriting:
- What does it feel like to see your screenplay come to life in production and on the screen?
- I have a great idea for a movie, but don't have the patience to write the script, and even if I did, the chances of the movie getting made are slim to none. What should I do?
- What are the best software(s) for writing a movie script?