The trailers tend to be loud and long, and only a few are that funny, interesting, etc. If they were aired at the end of a film, people would leave, and not stay to see them. I'm not fond of them, and don't know that many folks who are, and I'm not sure it would hurt film ratings to move them anyway.
Movie trailers used to be put at the end of films, that's why they were called "trailers". Should we go back to doing it this way? Perhaps that's down to last minute editing of the film, but I'd like to see more of that. Kind of like the extraneous webisodes that have popped up for Prometheus. That way you get content that gives you a feel for the film but, when the film reaches these all too familiar moments, the dramatic weight or the unexpected laugh hasn't been stolen from you because it was trailed so much.
You've already watched the movie. Why would you want to watch the trailer. It doesn't make sense. Most people just leave the cinema straight after the movie ends. Very few people stay for the credits. It's a waste of time. I like trailers before the movies- gives it more mystery and suspense.
Putting the trailers at the beginning is a good way to get people who are late into the movie. It's also a great way to keep people entertained that show up early. It's free marketing for people who make the movies, too. Everyone expects them at the beginning now, so there's no reason to change back.
Movie trailers should not be at the end of films because no one would stay to watch them. Movie trailers at the beginning of the movie allow viewers to see what will be in theaters soon, which allows movies to continue to be produced because there is demand due to advertising. While there are frequently too many trailers at the beginning of the movie, having them at the end would mean decreased awareness of future movies.