A lot of Hollywood is driven by demographics and economics.
First, demographics. Movies were originally designed for an audience of adults. The Production Code in the 1930s made sure that they were family friendly, but most the assumption was the the main audience for films were people 20 and above. There were children’s films, certainly, but most of those were Saturday matinee material – cheaply made. And the teenage market didn’t exist.
In the 50s, drive-ins created a teen market, but it was always minor. Again, the films for that market were cheaply made and sensationalistic, and often made by small independent filmmakers. Science fiction was a mainstay, enough so the studios dabbled in it, but studio SF always tried to be “scientific” and deal with themes other than straight adventure.
The turning point was Star Wars. Though Jaws created the summer blockbuster, it did deal with realistic adult characters. Star Wars was really a 1930s Flash Gordon serial with better special effects (the lightsaber battles were the type of thing you saw in the first Flash Gordon serial, though Flash used metal swords).
But studios noticed two things: people were seeing Star Wars multiple times, increasing the box office, and the major portion of the audience were teenage boys. They had a lot of disposable income (as ticket prices went up and there was more competition from TV) and were willing to see the same movie over and over again in a short period of time. The obvious conclusion was that if you appealed to this demographic, you’d have a hit on your hands. And this demographic had similar tastes to teenagers in the 50s – action/adventure, science fiction, and horror.
Teenage girls were original not considered part of the demographics, but Titanic changed that – it got the same reaction from them that Star Wars did for teenage boys. And now you have blockbusters for that demographic like Twilight.
Now, for finances. As the cost of making movies increased, studios became careful what they made. The more money you risk, the more conservative you are with your choices. So, since studios knew that these summer blockbusters made money, they made more summer blockbusters. It also made sense to go with a familiar property. If people recognized the title or characters, it made it that much easier to promote. In addition, action/adventure and the like do very well outside the US, since the language barrier isn’t a factor – dialog takes a back seat to the action sequences.
The blockbuster syndrome fed on itself. Older audiences were not as impressed by the teenage-oriented films, and that, coupled with the expense of theater tickets, meant they stayed away from theaters and became a smaller percentage of the audience, so it made financial sense to ignore them.
Ultimately, the studios make these blockbusters because the movie audience wants them. They use familiar properties because it makes it easier to create buzz. They use sequels because sequels can be counted upon to have a certain minimum box office (the percentage used to be pretty stable, too – and if you knew any sequel will do at least 50% of the box office as the original, then you can budget the film accurately).
The finances and the demographics give action films a big advantage in getting a green light. And films without action – not as expensive, but still pricey – don’t have a pre-sold audience from the outset.