Epic attempt 3: Common identifiers
A good epic builds up a sense of empathy and understanding with the setting, so that the viewer cares about what happens to the world after the climax. This is, of course, difficult to do in the time span allowed.
To efficiently (economically efficient) create a movie that evokes a sense of epic, moviemakers take a shortcut: instead of creating their own epic, they use an epic that someone else has already created, and make a movie out of it. The benefits are obvious: someone else has already invested the (artistic and/or economic) energy in getting the audience ready to feel that something is important. All they have to do is invest, tack on a surface story, and the underlying cultural knowledge of their target audience will do the rest.
When an audience feels that something has a history and tradition, half (or more) of the work of the movie is already done. For example, people have a cultural sense of gravity around "Rome" and "Batman" and "the U.S. Army." So, it is easy to create a 90-180 minute movie with a sense of "epic" about it. A battle fought for the future of Rome, Gotham City, or the salvation of the soldier's creed has canned significance. Prepackaged and ready to go. So, the writers don't need to invest half as much (or any) effort (money and screen time) in building up a plot that justifies a grand finale.
What does this mean? Firstly, characters do not need to be developed as much, in order to have the audience care about them, because the audience already knows that Gotham City (or whichever comic-book city) needs to be saved; the audience already knows Rome should not fall to the sinful barbarians; the audience already knows that. Secondly, the setting does not have to be developed as much. A few sweeping CG screenshots can handle it. If the audience didn't already identify with Gotham City, they might ask--why bother saving it (as Liam Neeson's character did in Batman Begins)? If the audience didn't already have a preconception of Rome, they might ask what made its hordes better than the barbarians. Precious effort would have to be invested to portray both sides of the conflict in a way that helped the audience understand the importance of the final battle. All of that can be saved to trim screen time down by going with canned settings.
The result of this is stagnation in art. When writers, directors, et al. can get the same "epic" result from a movie about Rome in 120 minutes that it would take a movie about a different, unknown place 220 minutes to convey, they will go with Rome, because it is cheaper. Unfortunately, tapping into this supply of "automatic epic" means that movies in this vein will have to conform to the audience's preconceived notions of the prepackaged concept--the audience's image of Rome, Batman or the U.S. Army. When you take prepackaged history to give your film gravity, you can't waste time challenging the core conception, or core misconception, that the audience brings to the film--that would defeat the purpose. So, the same themes about the same things keep coming back, again and again.
This is why, instead of writing new stories, the studios keep churning out old tales over and over again, in new form with new actors. If people go into a movie knowing it is a remake of something successful, they already feel that they know something of the characters: the characters are successful characters, with a history and tradition, and that automatically makes them have more of an impact. The studios kick out comic book movies, and show pages of comics at the beginning, along with a familiar corporate logo, to convey history and tradition. In 10 seconds, the audience feels, "This is all about something; something that has been going on for decades. How I want to see how it turns out!" without having been shown any actual footage. If they remake Gone with the Wind, or churn out yet another Shakespeare rehash, it is the same effect. The knowledge of previous success invests the story with automatic momentum--just the way George W. Bush grew up with an automatic headstart (a good metaphor for more than one reason, for most of the dross the studios churn out nowadays--solely economically gifted, and revered most by the unfortunate).
This is not to say that effort is not invested in producing a story. It is, and sometimes the result is a very interesting one. But even then, the amount of time that has to be invested is reduced--and the overall artistic experience (how enjoyable it would be to watch the full story) goes down.