They always have some dramatic-sounding chorus, and they always have plenty of percussion that gets faster and faster and builds to a crescendo towards the end of the trailer. Why are the people who make these trailers so unoriginal?
A lot of times, the actual music for the movie isn’t done until after the preview is already showing in theaters. Why hold up promotion for some silly post-production work (like the score) when you can slap a on generic theme and be weeks ahead of the game.
Because the mini-story arc you describe works to get people’s butts into the theater seats for almost any given movie; that’s why the trailers are all the same and the music is all the same: it’s a proven effective technique.
Now ask why all scary movies have the same minor key strings or synthesizers playing as a means of creating tension when there is very little action on screen…
Their job is to create something that gets people to talk about the movie and bring people into the theater. They must know what they’re doing, since the trailers get people talking about the movie and going to the theater.
Making an accurate description of a movie is far down on the list of what’s important to achieve the goal.
Note, too, that trailers are made before the movie is completed. No matter how accurate they try to be, the post production process may completely change what end up on screen. Look at Justice League – they had trailers out last spring, but, after Wonder Woman, Warner decided to reshoot scenes to feature her more prominently. There is no way that could be reflected in a trailer made before Wonder Woman movie came out.
The main reason trailers use the same music is because they are made before the film is scored. There’s no point in asking the composer to create something special for the trailer, and there are many advantages of using music that people already know.