There are a number of reason why it is so hard to sell a screenplay. They are logical, real and completely understandable if you think about them. There is one reason that you will be told that isn’t real. There is no club. Studios and producers are not willing to lose money to maintain a small list of writers. If there is money to be made they will hire whoever they need to make that money.
Some immutable facts about selling a screenplay.
There are only a set number of viewing opportunities for films in any one year. For simple calculations we will say that it is 10,000. If there is one film, it will get 10,000 views. If there are ten films each would get 1000 views. I know this isn’t a real assessment, but you get my point. If there are too many movies, no one makes money. By limiting the number of films being made the producers increase the scarcity which increases the value of each item (basic marketing).
So since they are limiting the number of films being made, they become more picky in what they select to make.
Sequels and remakes.
A sequel or remake has a built in audience. They also have free built in advertising. Once a studio or producer announces that they are marking a new sequel or remake everyone talks about it. The media will start with the “will it be as good as the original” etc. This makes these films attractive.
The other things that makes them a good choice is the fan base. If people liked the first “blah blah blah” film, then they will go see “blah blah blah 2”. The only thing the marketing has to do is let these people know, if the media hasn’t already, and try and attract people that missed the first.
Existing IP (Intellectual Property).
This is when they make a comic book movie or a movie based on a book. In the 1990’s it seemed that every film was based on a Stephen King novel. This is very similar to the sequel reason. The books have a built in audience. The Harry Potter books had queues well before they became movies. Buying the rights to those books was a no-brainer.
If Christopher Nolan wants to make a spec script about bird watching. It will be taken seriously as an option. It may not get made. But they will listen. With all the previously successful creator pitching work, there is even less space for a new voice to be heard.
So what can you do?
First thing. You have to be as close to perfect as you can. This sounds like stupid advice. Like someone saying “go write a hit song”. But you have to have the best product you can. There is no reason for a producer or studio to take your second class effort and work on it for you, or with you to make your dream come true. You are competing with all the franchises, remakes, previous known works and previously know writers. It is possible, but focus no being good.
The other option is to build a reputation in the minor leagues. By that I mean look for low to no budget producers that needs a script. A good script will shine through, no matter how low the budget. This will show that you can write to a budget. That you understand how to work within constraints. It will get you some credits on your resume.
Why write a screenplay for free (or at least cheap)
A good screenplay stands out in a bad film. There is also the chance that the producer or director have talent and they will get bigger budgets and they will want to work with a writer they know and that can work inside constraints.
In a nutshell it is up to you to build a reputation so a sale comes to you. You have to make yourself competitive in a competitive market.