So what ghostly experience did King endure that triggered the novel? Presumably the exact stuff we know from the book or movie didn't happen (no salami-skinned Susan soaking in the tub, no Kool-Aid Man's burst hemorrhoid on the elevator, no BJ and the Bear Suit), but the place had to have some history that inspired the story, right? Nope! King just liked it as a setting because the corridors were spooky and he had a nightmare while sleeping there. Then, once the book and film became big hits, the Stanley took full advantage. They even ran a horror film festival for a while, which would host an immersive game, putting participants through the paces of hunting kidnappers and occultists and solving mysteries around the grounds.
Then there are the ghost tours, of course. They even had that hedge maze installed (yep, they built it after the movie; the hedge maze isn't even in the book). Soon, staff and guests started insisting they'd had real ghostly encounters. See how that works? The hotel inspired King, who inspired guests, who inspired the hotel to start crafting their own legends. At some point it just becomes more profitable to pretend that the fiction is true, and has always been so.