Trailers routinely show images from a film's final stretch, including moments the film itself treats as revelations. This is a tested decision rather than an accident of editing, and it follows from what a trailer is measured on.
A trailer is sold on intent, not on mystery
Marketing campaigns are evaluated by whether people say they plan to see the film. That question is asked repeatedly of sample audiences, and the answers shape the cut.
Clarity performs well against that question. A viewer who understands what the film is, what it looks like and how large it feels is more likely to commit than one intrigued but uncertain.
Withheld information reads as vagueness in a two-minute format. The audience does not experience it as suspense; they experience it as not knowing whether the film is for them.
The most expensive images arrive last
Large-scale sequences are usually placed in a film's final third, because that is where the story concentrates its scale. Those shots are also what the budget was spent on.
A campaign that avoids them is left promoting a film with its most persuasive material hidden. Studios are unwilling to spend heavily on production and then decline to show the result.
The pull is strongest for films competing on spectacle, which is exactly the category where late reveals are most likely to be shown.
Attention is decided in the first seconds
A trailer attached to a video is skippable within a few seconds, so the opening has to establish stakes immediately rather than building toward them.
Leading with a striking image means leading with something from late in the film, since that is where striking images live. The format forces the sequence out of order.
Shorter cuts made for feeds compress this further, sometimes to a handful of shots. At that length there is no room for anything other than the best material.
Familiarity turns out not to hurt enjoyment
Audiences often report enjoying films they already knew the shape of. Knowing what happens and watching how it happens are different experiences, and the second survives the first.
Studios have observed this in their own testing and behave accordingly. The perceived cost of spoiling is lower than the measured cost of confusing.
The complaint about trailers is real and is made by a group that would have bought a ticket regardless, which is why it does not change the practice.
Filmmakers have limited standing to object
Marketing sits outside a director's contractual control on most productions, and the campaign is built by a separate department with separate targets.
Some directors negotiate approval, and a few release deliberately opaque campaigns, which is possible only when the name itself sells the film.
For everyone else the trailer belongs to the people paying for the release, and it is cut to serve their measurement rather than the film's structure.