Legal thrillers set in American courts follow procedure closely, and the accuracy is not pedantry. The rules supply the structure that makes the plots work.
Procedure creates deadlines
A trial calendar imposes dates that nobody in the story controls, which gives a novel a clock without the writer having to construct an artificial one.
Filing deadlines, discovery cutoffs and the date a trial begins all press on the characters regardless of what they would prefer to do.
That external pressure is what separates a legal thriller from a courtroom drama, and it comes free with the setting.
The rules restrict what a hero can do
A lawyer protagonist cannot simply act on what they know. Evidence must be admissible, privileged information cannot be used, and misconduct risks the case and the license.
Those constraints are more interesting than physical danger, because the character is boxed in by obligations they have chosen to accept.
Readers understand the constraints well enough from public familiarity with courts, so the writer can rely on them without explanation.
Discovery is a reveal mechanism
The process by which each side must disclose material to the other is a purpose-built engine for delivering information at a controlled pace.
A document that arrives in a late production changes everything the protagonist believed, and the arrival is procedurally plausible rather than convenient.
Writers use it because it lets a twist land without a coincidence, which is the hardest thing to arrange in any mystery.
Cross-examination is a scene format
Questioning under oath is dialogue with rules, a defined objective and an audience judging every answer, which makes it naturally dramatic on the page.
The witness cannot leave, the questioner cannot lie about the record, and the judge can intervene at any moment.
Few other settings provide a conversation with that much built-in tension and so little need for staging.
Accuracy protects the illusion
Many readers of the genre work in or around the courts, and errors in procedure break their attention faster than an implausible plot would.
Writers with legal backgrounds dominate the category partly for this reason, since the working knowledge is difficult to acquire secondhand.
Where fiction does bend the rules it usually compresses timelines rather than changing what the rules are, because the rules are what the story is built on.