In short... money.
Corporations want to maximise box office. Hell, with movies costing £200 million+ to make, studios NEED to maximise box office. We're at a point now where a movie needs to make north of $600 million just to break even.
How do you do that? Well, your movie must appeal to as many people as possible. You can't afford to offend any significant demographic, and you must incorporate something for all demographics. So for instance, the Star Wars prequels. They stuck Samuel L Jackson in as a Jedi, because they need to appeal to black people. Never mind that Jackson is a shouty angry type of actor who does shouty angry roles, and Jedi are nothing like that - he wasn't cast for his acting ability, he was cast so black people would pay to see the movie.
Your movie must serve as an advertising platform for spin-off media. So it has to contain elements that can be turned into toys. Remember Padme? Stood around saying bland things, wore all sorts of different dresses? That's so they could get girls to buy Padme dolls and a dozen different outfits to dress her in. Ever wonder why every single character in a Star Wars movie gets a background and a name? Even the background extras who walk past in one scene? That's so dolls of them can be sold, and spin off novels written featuring them.
The story has to be structured in such a way that it can be easily turned into a computer game, or several computer games. Ever notice how movie action scenes are looking more like computer game action scenes lately? That's why. And there must be potential for the spin off comic book series. The spin off TV series. The McDonalds meal deal tie-in.
Most importantly, your movie must not end, as such. The story cannot come to a conclusion - it must be left open for the sequel. Your story must span at least three movies, preferably four - and the gold standard is that your story allow an ongoing story with no end point. So instead of an ending, your movie must merely resolve today's problem, whilst clearly establishing that there will be another problem tomorrow, and tomorrow, and tomorrow.
And that's why they are so generic. Because every expensive movie made is written by a committee, and all of them according to the same formula, for the same ultimate goal that the studio is aiming for.