The movie industry constantly changes, adapting to changing times. Not every "big" movie is bad, but it seems like most are average at best.
In the 50's and 60's the youth/teen market was targeted with "B" movies. Big budget movies were aimed at older audiences or all ages. They were a response to the growing influence of TV, which allowed people to stay home and be entertained.
TV eventually won out, and that was furthered by videos and DVDs and over the years, movies stopped targeting the older demographic as their prime demographic. Teens and twenties became the prime demographic because they went to the movies to socialize regularly.
Now, the older demographic and the market for more intelligent movies would argue that they stopped going to as many movies because of the dumbing down of movies, not because they got older, started families, changed the way they socialize, etc. The truth is probably somewhere in the middle.
The B Movies still appeal to the younger demographic, except they now get the big budgets--hence superheroes, sci-fi, etc dominating the current movie market. With bigger budgets comes bigger expectations... the stakes are now higher. So you get sequel after sequel, with executives not willing to take chances with huge budgets. The explosion of CGI has made the ability to make these types of movies more realistic has also been a factor.
The other issue is the globalization of the movie business. It's always been a part of the movie market, but it's become a huge part of the market. Which movies are going to be more universal and play/market in the US, Europe and Asia? X-Men, Godzilla, Transforners, Aliens 15... or Brokeback Mountain, 12 Years a Slave, Milk, Moonlight?
Much like TV, where the major networks are full of drivel, and most of the cutting edge and best TV is on smaller networks (or Pay TV), much of the better, intelligent films are indie films. Which thanks to technology, are much easier to make with higher production values than it used to be.
There are exceptions, of course, but there are still a lot of really good films being made today, both indies and studios. But they don't get the same exposure.
Of course, it could also just be the dumbing down of America, where science, intelligent discourse, etc. are frowned upon. And shows like the Academy Awards are called elites handing each other awards for bad movies no one watches.
