The primary distinction is that in a screenplay, a writer must do that accomplishment faster than in a novel. A feature-length screenplay is typically 110–120 pages long, although a novel might be several hundred pages long. If that's the case, you may start outlining your script. Otherwise, your narrative would be better suited for a novel. Either way, good writers know that the best stories are never fully realized on the page or screenplay stage; they need to be developed through revision.
Writing a script is much more limited in scope than writing a book. You can't deviate too far from the original story idea without causing problems for the film industry when it comes time to make the movie. The writer of a screenplay has very little freedom over the material; instead, he or she is responsible for bringing the story forward in a clear and concise manner that someone else can interpret later.
Furthermore, screenplays are not considered novels in terms of style or content. While a book can explore different perspectives and ideas through diverse characters or situations, this isn't possible with a screenplay. Writers are expected to include everything necessary for the director to tell the story effectively and appeal to viewers across the world.
Finally, books allow for significant expansion after publication while scripts cannot. Once a book has been written, it can usually be published any number of times without changing much other than the font size on the page.
Because just one out of every ten scripts is produced, selling a screenplay is more difficult than selling a novel. While the main distinctions between a novel and a screenplay are the amount of pages and the quantity of written words, the reader evaluates the plot equally. In fact, it's even possible to write a bad screenplay.
The average length of a movie is 120 minutes. This means that on top of being less time than a book, movies must make their stories interesting in fewer hours. The number of people who read before they watch films is declining, so there is more demand for compelling television shows and video games. If you want your work to be seen by millions, consider writing a movie instead.
Writing a book is expensive. You need paper, ink, printers, and binding services if you want to print something yourself. It's easy to underestimate the cost of publishing a book, but even a free self-published book requires some sort of investment. Websites like Amazon give the illusion that everyone can be a publisher, but only professional publishers can afford to release books without making any money off them first.
Screenwriting is its own medium with its own set of rules. There are many similarities between novels and movies, but also important differences which mean that learning how to write one isn't enough to write the other.
If you like books, you should definitely write one instead of a screenplay. Novels are the ultimate form of art—you create a book, and that's it. It's on a shelf with your name written on it. Screenplays, on the other hand, are just one step in the long process that leads to the final creative form: a film. Even if you write the most amazing script ever conceived, it means nothing until someone else turns it into a movie.
As for why people choose to write novels instead of screenplays: it's easier said than done. Writing a novel is a huge project that may or may not be successful. There are so many factors at work here that it's hard to know exactly what will sell well and what won't. For example, readers may love your character arc and want to see how they end up, but they may hate the actual story line itself.
Writing a screenplay, on the other hand, is much more of an individual pursuit. You're working within a limited scope and there are clear success and failure markers along the way. If you write a bad scene, you can always fix it in post-production. But if you write a great scene that doesn't get used, you've lost out on opportunity revenue.
Of course, these are just generalizations. There are exceptions to every rule. But as far as we know, this is how things work in the world of literature and cinema.