Writing a novel is much different than writing a screenplay. With a book, you have to actually describe the world for the reader, inserting crucial details along the way to create a sense of place; this makes it easier to imagine the world your characters inhabit.
A screenplay allows you to breeze over all that mumbo-jumbo with pointed little descriptions like “dark room” or “big guy”. It’s up to other creative personnel involved with the film to bring the writer’s world to the screen. If you’re lucky they won’t change too much (spoiler alert: everyone’s unlucky in this regard).
There are bonuses and drawbacks to writing in either form. Screenplays are definitely faster since you’re not messing with all those pesky words. However, you run the risk of improperly conveying your ideas to a producer or director if too much is left out. It’s a tough balancing act that a lot of people tip one way or the other, resulting in a rejection. Novels let you build a world into which readers can escape from their everyday lives. They are a commitment: screenplays can be read in 1-2 hours, but novels take longer, forcing you to stay inside the pages for days or even weeks.
Movie audiences are also looking for different things than novel readers. It is sometimes okay to get through 100 pages of a book with no huge action as long as the world is so richly detailed you forget you’re bored. Movie patrons would have left their seats after half an hour.
Both formats are fun for their own reasons. I prefer novels at the moment because I just finished writing one and it’s a blast. Now I get to take a break and help a friend write a screenplay, so I’ll get to experience the best of both worlds; the long and short of it, so to speak.