Fan F goes to Lawyer L, and sues. Now, even if Lawyer L knows there is no case at all, she might well decide to go for it anyway, in hopes that Franchise Z will offer a settlement just to make Fervent Fan F go away. They have just, in effect, been sued for violating their own copyright. Unsolicited media tie-in manuscripts don't get read. The franchise holder and the publishers thus avoid a very real and highly probable danger by turning their back on the rather hypothetical and quite highly improbable hope that Fan F has written something vastly better than anything they could get from their stable of professional writers, and something that exactly fits in with all their future plans for the franchise.
Further variations cover the writing of scripts for TV and movie series.
It's not fun to tell people the bad news, but there it is. I wrote a lot of books that were all my own, and developed a name and a reputation in the science fiction field. By so doing, I proved that I knew how to write a commercially successful book, and proved that I understood the business. I had to write my own books before they came to me and asked me to write their STAR WARS and Robot novels. This brings me to another topic.