I dislike that too.

It results from market analysis (and I would guess it probably reflects the trends of the "spoiler culture" spreading and being so popular- heck, nowadays some people have bought a book and they still go on forums asking people about it, and what to expect from it and what is good and what isn't etc. before starting it! Next, some will start buying a book to have someone read it for them!).

Marketing depts of studios discovered it's better for them when they show a greta deal about the movie in the trailers (how much, and what type of "spoilers" are included, eg: outlining the story in full, focussing on impressive visual scenes etc., showing or not the iconic moments or the big moments of the movie, trying to to make people think it's more this-type of movie than what it really is etc. varies with genre, movies and target audience. Most trailers for romance/romantic comedies outline the full story and present all the players and main situations, for example). Roughly, with the people who list trailers quite high on their list of what might decide them to pick a movie over another, it turned out they say "spoilerish" trailers have better odds of convincing them to see a movie than something that is more like a teaser/introduction to the beginning of the movie only. It's them the trailers target.

They're calculated to the minute detail - and most trailers are done by the same small group of editors who specialize in that in LA, which is why they are often so similar (and the instructions they get from marketing folks to work look like a cooking recipe). There's a guy who is the specialist of those trailers that outline the full story, and these guys who are kind of spin doctors to make a movie appear to be whatever the marketing people think it should be to attract people (one of them I know of specialize in comedies, the other in action flicks).

It's annoying, but it seems a great deal of people who hate trailers that reveal too much (or totally mislead) also don't think trailers are a big factor in what make them go to theaters, so the studios don't tailor their trailers for them (they reach them with other types of publicity) and rather for the audience which trailers influence.

Trailers annoy a lot of people in the industry too but marketing people are in charge of that part of the business and they have the stats to prove they're right. The trailers are mostly conceived with the US market in mind too.

There are still some "old-fashioned" people when it comes to trailers (George Lucas notably, and unlike 99% of directors who have zero say in that, he does have control on his trailers, as he's the boss of the production company) and you still see some "old style" trailers for indepedant productions (though when they're bought by a distributor, they often a more "commercial" trailer edited).