And a lot of the time, I don't think they even finish the final edit until right before the premiere. I'm pretty sure I remember hearing stories about editors finishing and handing the film or whatever they're on in the digital age to a person, who literally drove it over to the premiere and started playing it.
Robert Wise had to release
Star Trek: The Motion Picture to theaters before he was finished editing it due to the rigid release date, and he requested permission to finish the edit and have the final cut shipped to theaters to replace the one already showing. But Paramount wouldn't shell out for it, so he had to wait 22 years to see his director's cut finished.
The only thing that is a little irritating is when they create fake scenes just for the trailer
I don't see them as fake, I see them as symbolic. The point of a trailer is not to be an accurate document of the film's content, but to create interest and curiosity about it. If a scene created just for the trailer conveys the tone and substance of the movie, it's done its job, even if that exact scene won't be in the film.
or when the trailer promises one tone and the final movies is totally different. One of the later that stands out for me is Operation: Elephant Drop, which the trailers made look like a silly comedy, but then the final movie was actually pretty serious, with the only funny scenes being the handful in the trailer.
If
all the trailers do that, then it's misleading, but the standard formula used by the main couple of companies that produce movie trailers is to have each one emphasize a different facet of the film. For instance, the first teaser trailer is often more vague and ominous (largely because it's released before the CGI is finished so it tends to omit big critical scenes), the second is usually more straightforward and specific about the plot, and the third often emphasizes the humor and pithy dialogue. So a single one of them may not represent the film's tone, but it's not meant to be the whole picture, just one part of a triptych.
For instance, for a rather drastic example, the teaser trailer for the
Ghostbusters reboot (the one retroactively subtitled
Answer the Call) was so serious and ominous that if you weren't familiar with the franchise, you'd never have been able to tell it was a comedy. But the subsequent trailers showed its comedic side clearly.