Back when I was temping, I spent a couple of months working in Cable Development. It was a great job with great people, and I had a lot of fun there. (Then the person who actually had the job came back, and he liked his job as much as I did.) One of the scripts I read had characters knocking each other out by pinching the area between the neck and shoulder for a second or two. Yes, the writer used the Vulcan Nerve Pinch. Three times. Unironically.
It’s easy to see something become a trope and then get used so often that everyone just accepts the world works that way, even when it doesn’t. Mythbusters has whole episodes about it. Writers are really good at getting professionals (especially medical and legal) to say, “well, I suppose it might work like that,” and then the action becomes an established shortcut. We LOVE shortcuts.
Sometimes there’s a spiral effect, where “it might work like that” becomes A, and since A is now established fact and B follows A, B will also work. For instance, it’s become very popular in TV and movies to note that hitting a person in the head can render them unconscious. Aside from what the linked Cracked article notes, the amount and placement of the hit has become more and more lax. I remember seeing, in the same week, Dollhouse and Law & Order: SVU both incapacitate a healthy, full-grown adult woman with a smack to the face.
Sometimes it’s like using something as a catchphrase, when it actually has a specific meaning. A few years ago, there was a short-lived procedural that started out by explaining this character back story:
There had been a robbery at a convenience store. The owner handed over the money, then followed the robber outside with his own gun and shot at him, missing. The robber turned and shot the owner to death from about a half a block away. The weapon was never found. A suspect was tried and convicted. A few years later, he was exonerated based on DNA evidence.
In this case, “DNA evidence” is being used as a catchphrase to explain why the suspect was cleared, but has lost its original meaning. (Re-read that back story and tell me exactly when the robber would have left DNA.) This makes actual prosecuting attorneys insane when juries demand DNA evidence and it just doesn’t exist.
Sometimes it’s just about creating conflict or heightening tension. Contrary to what TV shows would tell you, the FBI is not a super-police force that swoops in and steals interesting cases from local cops. They have their own, distinct set of responsibilities. The work may overlap, but the job of the FBI is not to make life harder for police officers. Likewise, you typically cannot get the death penalty for simple murder. (Although, since the police are legally allowed to lie to you, in an interrogation they can probably say that you’re eligible for the death penalty. But since the cops in TV shows are usually the heroes, they rarely outright lie to get a confession.) If you want a crime in your story to warrant the death penalty, you might have to add a little something to it.
Sometimes it’s about simple laziness. Not all states are California. Not all states have three strikes laws. Research the laws in the state you’re writing for. Even if you’ve never been there. It’s not hard.
Of course, sometimes you don’t have to care. A bullet is going to blow this truck up, killing your bad guy and propelling your hero to safety so he can impregnate his girlfriend with his thoughts. Fine. Great. You have fun. Your audience will probably have fun, too. But if what you’re writing is supposed to be set in the real world, you might want to double-check it.