The most obvious answer for me would be that it's much cheaper to write a book than make a film, and you have to rely less on other people so there's less to go wrong (in theory).
I think film and novels are completely different things. A film delivers visuals and speed, while a novel can do depth and insight. Every year or two, someone arrives here with plans to write a novel, but only references films in connection to it. I always think "That's not going to work". I don't think you should write a novel solely because you can't make a film of your idea. There has to be more.
Films have also become much more risk-averse, as budgets have grown, and franchises with fairly predictable storylines have become the norm, which isn't very good for more imaginative writers. I recently realised that the things I've liked about the Marvel films have been the things that break the "rules" of the subgenre. A film like Captain America: The Winter Soldier can contain loads of interesting ideas - but sooner or later it has to default to the safe option of big special effects and magic people punching one another without anyone at serious risk of death, because fundamentally that's what the industry requires. A book doesn't have to do that and, provided that it's consistent, it can go to a lot of different places.
And then you get "imagination-heavy" books like Titus Groan or Dune, where part of the pleasure of reading is imagining the places depicted, and a film has to work very hard to show anything as good - and may then still disappoint in terms of the decisions that the designers make.