So, as someone who has written a lot of fan fiction in my life, I'll answer this. It's fun. It's a hobby. And for me, it was a way to practice writing with a story and characters I knew and loved. I loved to write and had these stories in my head and needed to get them out. It was fun writing and posting stories, and I met a lot of friends in the process. Eventually, I did write original fiction, but guess what? It wasn't nearly as fun as writing fanfic.
Also, count me in as someone against using generative AI in any kind of storytelling. The whole fun of creating is
creating.