Having originally moved to L.A. to pursue screenwriting (you're shocked, I know), my shift into magazine articles and novels was gradual but probably inevitable because (whispers) I love the written word.
It's not that I don't also love film and TV, but screenplays are a mere component of their creation. Screenplays are meant to be a blueprint, a jumping-off point for directors, producers, soundtrack teams, cinematographers, actors, and possibly entire teams of other writers to interpret and add to. Sometimes the results are brilliant; other times the original intent can get lost in translation.
With a novel, the writing is the final production, all rolled into one. You're created an entire world with all its components in place, all the marvelous, wide-open possibilities, special effects included, and handed them in tact to readers, from your brain to theirs, as if by magic.