And so, having found that origin stories are a handy narrative tool for kicking off a franchise, Hollywood decided that every superhero movie should be an origin story, dropping our spandex icons into a Groundhog Day loop of childhood traumas, first kisses, and clumsy jumps off high roofs. The intro portion that used to take 10 minutes at the beginning of a movie is now filling entire movies — franchises, even. Consider this: In 2002, Sony gave us a spiffy version of Spider-Man’s origin, complete with the radioactive bite, Peter’s crush on Mary Jane, and Uncle Ben’s death. A mere 10 years later, we were already hungry for a step-by-step refresher course in The Amazing Spider-Man. And this time, the filmmakers decided to stretch Spidey’s origin out even longer — when the credits roll, we haven’t even met Mary Jane, one of the most important character’s in Peter Parker’s world. She’s holding out until the sequel.
Likewise, in 1989, Tim Burton’s Batman barely glanced at Bruce Wayne’s backstory. But when the series relaunched in 2005, Christopher Nolan devoted all of 2005′s Batman Begins to reintroducing the world to a darker, more Bale-ful caped crusader. And two movies later, Dark Knight Rises couldn’t help but to fold in yet another origin story: Robin, the cop-turned-crimefighter. The Avengers movie was almost entirely the story of the crew coming together, Captain America shows Chris Evans’ full transition from 80-pound weakling to Nazi-trouncing Abercrombie model, and X-Men: First Class literally rewound the series back to its Muppet Babies days. (One notable exception: Thor, which whipped through a relatively quick set-up to make way for hammer time.)
It's a thoughtful piece. However, I'll offer a more simplistic answer: the real reason is sloppy writers. It's so much easier to write an origin story. To create an actual story instead, requires talent and the hacks writing don't have it. Not at all.
So you take a 'this is how it started' and it is much harder to screw that up. Audiences are also more patient because they know you're going to get to something good at some point. So they cut you slack that they wouldn't otherwise.
Some films are so bad, half-way in they still have nothing to offer.
The origin story is like a cookie cutter for bad writers and it's a cookie audiences will gladly eat because they've sat through too much crap -- Batman & Robin, for example.
A Tim Burton is a genuis. He can tell stories. A Christopher Nolan has to have a cookie cutter script to work with because he's a hack.
Closing with C.I.'s "Iraq snapshot:"