Storytelling was recognized as formulaic as far back as Ancient Greece by Aristotle in his book Poetics. He knew then that most people like their stories to end up with the suffering hero redeemed, the villain punished for his misdeeds, forbidden love triumphant, etc. Therefore, that's what the moviegoers have paid for year after year, and that's what Hollywood continues to deliver today. It sells.
I think the problem is pretty simply a glut. Thanks to modern media and communications, and extra thanks to cheap filmmaking gear, everyone is constantly exposed to endless variations and combinations of these stories. Flip on the TV and there are dozens of movies waiting to stream into your brain. Even if a few are decent, most don't even rise to the level of Sharknado or Snakes on a Plane. And with so many choices, we lack the editorial reviews and critics we might otherwise use to keep out the dross.
When you see a movie that's truly new and novel, it sticks with you. Sometimes its a good story or came from a good book, sometimes it's a great actor, sometimes it's a new special effect or cinematography trick, or sometimes it plays on our childhood memories. Of course success quickly breeds imitation, and within months there are 58 variations on the theme, adding to the glut. And when the producers tire of the imitators, they release an official sequel or three, and eventually add a "reboot" or "remake" of the originals that captured our imaginations so long ago. They snazz it up, apply extra-modern graphics, bring in Daft Punk to record the soundtrack, hire sexy-fresh new kids to be tomorrow's stars, and retell the same old stories.
Spielberg knows his problem is not that his next movie will have trouble competing with the current releases. It's that he's really competing against our fondest memories of classics such as Raiders of the Lost Ark, Shindler's List, Star Wars, Jaws, Casablanca, Snow White, and Toy Story, all of which are still busily crowding themselves onto our cable channels and Netflix queues. So other than the fact that he's got a billion dollars in the bank already, he's completely screwed.