Harry Potter fans force relocation of £430m undersea cable to avoid Dobby’s ‘grave’
(www.theguardian.com)
There is fan merch of The Room. Theme parks don't need a popular franchise connected to be popular, so long as there's a rollercoaster and something fun for the whole family. People will happily spend money on nostalgia and fitting in with others, or on gifts for someone you think likes the thing.
All of this is, once again, an appeal to popularity. It has nothing to do with the quality of the books. And you saying the book isn't bad despite bad plot, world, characters and implications just condemns your judgement.