Why don't people who claim to want to write a book...write that book? Mostly, I think it's about unreasonable expectations of what "being an author" is and what writing a novel entails. I'm not going to get into the selling of the novel because that's a leg of the journey beyond this week's blogging prompt.
Unless you live with an author, your exposure to what it means to Write A Book has probably been through the fictitious portrayals of an author's life in TV and movies. It's a fun and flexible occupation for a character to have, but like most occupations portrayed on TV, the actual act of doing the job isn't anywhere close to reality. Rick Castle, Jessica Fletcher, Rory Gilmore, they spend five seconds on screen every fifth episode "writing." Sure, there's a reason for that. Watching an author do their job would be some dry, dry, dry TV. Type, type, type, swear, talk to themselves under their breaths, get up, pace, get something from the fridge, write some more, get up, get coffee, erase everything they'd written, let the dog out, scan social media, make a snack...you get the idea.
Folks who "want to write a book but..." rarely stop to think about a novel being the longest term paper they've ever written. If they did, they'd find a local author, hand them a bottle of whiskey, and walk away...after buying two copies of every book ever written by that author, naturally. 🤣