If someone sends a movie studio a movie script they wrote and the studio likes it, they presumably like the idea, not the exact setting and character names. So why not change those and call it their own? The end result, the published movie, will be quite different from the script anyway. So why would they pay for it?
(From this answer it seems like there are people who read scripts, so it seems like it's not impossible for this to happen, for someone to sell a script.)
Because litigation is expensive, both in terms of actual dollars but in terms of reputation.
Harlan Ellison's threat of litigation was enough to get him an undisclosed cash settlement and an acknowledgement credit on James Cameron's 'Terminator,' based on a short story and an episode of Outer Limits that Ellison wrote three decades before the movie. Apparently, Cameron considered this a nuisance suit but was pressured by the producers to settle.
Film producers want to make money and lawsuits can consume it in vast quantities. If they made it a practice of infringing on other creatives intellectual property they'd spend more time defending themselves than making movies, I suspect.
For the sake of argument, assume that a film maker did steal an idea from a script, they'd need other writers to adapt that idea into a new script. It would be in those writers own interest to expose the practice since their scripts would be even more vulnerable to infringement, because they are professional writers who know how to come up with good ideas for their stories. They'd be undermining their own possibility of success. Since most people act in their own self interest, I don't see this as a standard practice.
Also, many scripts are sold and stories optioned that never get made into movies. For movies with budgets in the millions a few tens of thousands to product the project from easy to avoid litigation seems like a no brainer.
I don't doubt this kind of infringement happens in Hollywood and other businesses. I doubt it is a rampant practice.
I see what you're saying, maybe, but if you change the setting and characters then all you have is a premise that falls within a genre. There are a limited number of each of those but an infinite amount of characters, environments and interactions that can occur within said genre/premise. The studio pays for those fine details. They pay for the imagination that built a solid script. Of course they wouldn't pay for someone to say, "I have an idea for a movie! It's a horror movie set in England. Where's my check?"
