Here's why:
* A trailer is a promotional tool: It's designed to generate interest and excitement for the movie, and to convince people to go see it.
* You need a finished movie to make a trailer: A trailer is made up of clips from the actual film, along with music, sound effects, and narration.
* A trailer can't be made until the movie is finished: It's not possible to make a trailer until the movie is shot, edited, and scored.
There are a few exceptions to this rule:
* Teaser trailers: Sometimes, studios will release a teaser trailer for a film before it is finished. These teasers are usually very short and don't reveal much about the plot, but they are designed to generate early buzz.
* Marketing materials: Some promotional materials for films, such as posters or online ads, might be released before the trailer is made.
However, the general rule is that the movie comes first, and the trailer is made after the movie is finished.