Why do writers and movie makers so often choose nature as their subject?
I don't know why fiction writers often choose nature as their subject. I'm not even sure I agree that they do often choose nature as their subject. However, I have a pretty good idea why movie makers choose nature. It is because it is so much cheaper to shoot pictures outdoors than indoors. They don't need to build sets and they don't even need artificial lighting. They can also get spectacular photography at little expense. It seems especially the case that foreign filmmakers tend to shoot a lot of the footage for their films outdoors. They almost never have big budgets to work with, as they do in Hollywood. All they need is a little love story that takes place between two people in a beach setting or somewhere out in the country. Beaches are extremely popular because they can show the principals lightly clad. Typically they seem to have only one indoor setting. That is a house they rent from the owners. (The movie makers might not even own their own cameras, since movie-making equipment is easy to rent.) Another reason that beaches seem so popular in low-budget foreign films is that there is no cost for costuming. All they need is a few bathing suits, and the actors and actresses can bring their own. (An excellent example of a low-budget French movie shot at a beach is C'est La Vie by Diane Kurys.)
People go to the movies for escape. Pictures that are shot out in the open provide visual escapism easily. Nature provides infinite photographic opportunities--and these pictures of beaches, lakes, waterfalls, forests, prairies, etc., etc., are escapist for the average moviegoer who lives in a crowded city or a dreary suburb. It may be that the movie makers' interest in nature has influenced the authors of novels and short stories, because, for one thing, these authors would dearly love to have a producer buy the movie rights to one of their works. Furthermore, novel writers and short story writers are influenced by what they see on the movie screens just like everybody else. Motion pictures have had a tremendous influence on modern fiction.
Movie makers relied heavily on sound stages in the old days because of all the problems connected with lighting and sound recording. The movie cameras were big and heavy, extremely difficult to move around. Nowadays they have lightweight cameras and film that can be used in any kind of weather conditions. The dialogue is usually handled by dubbing, so that two people can appear to be talking to each other in a noisy setting, such as on the brink of Niagara Falls, and everything they say can be clearly heard. The actors read their dialogue into microphones at a studio and the dialogue is dubbed into the movie soundtrack by a technician.
Narratives that concern nature often explore a particular question regarding identity: What is a person outside of society?
When the standards of personal comparison are no longer in place, what does it mean to be a good or bad person? Without other people around, the relative nature of social values also tend to diminish or disappear. Thus, when characters are situated alone in nature interesting perspectives arise regarding what the individual is, as a person, and what he or she values in an absolute way. This kind of narrative exploration takes place in the television show, The Walking Dead, where characters are challenged by circumstances and find themselves adopting an entirely new moral code in the face of severe situations. We also see a similar theme in Deliverance.
In stories that situate multiple people in nature - outside of society - comparisons between individuals become stark and exaggerated. In a group of three people, someone who is only slightly "bad" can become the worst of the bunch. Writers sometimes explore the effects of this dynamic on a character's self-concept. A good example of this can be found in Lord of the Flies, where two mostly normal boys (Jack and Ralph) become opposites and enemies based on differences that begin small and grow in the isolated social environment.
Also, survival is a very basic and universally understandable concept. Survival is the basic narrative conflict of all species, in a way, and so functions as the backdrop to many stories.
Of course, different authors will have different reasons for posing their stories in nature and/or using nature as a subject of narrative.