The following musing is only one of these attempts to understand what's going on. Let me first state that I've read fan fiction, edited fan fiction, and even written fan fiction on and off for most of my adult life. There's nothing wrong with fans exploring the world of a beloved story. But recently, many original and franchise books, movies, and games have shown a certain resemblance to fanfiction and other derived works. This is not a description of quality, but of attitude. And it all comes down to what's being celebrated, the work or one's self.
What is the difference between homage, pastiche, and fan fiction? And why does it matter?
Homage: Something created or done in honor, admiration, or celebration of someone or something
Pastiche: A work of visual art, literature, theatre, or music that imitates the style or character of the work of one or more other artists. Unlike parody, pastiche celebrates, rather than mocks, the work it imitates. In literature usage, the term denotes a literary technique employing a generally light-hearted tongue-in-cheek imitation of another's style; although jocular, it is usually respectful.
Fan fiction: Typically used to denote derivative works written by fans, the original definition from the pulp era is "fiction about fans, or sometimes about pros, and occasionally bringing in some famous characters from [science fiction] stories". As this is the particular group of stories which has given rise to the Mary Sue, the self-insert, the author avatar, the original character, shipping stories, and the meme fic, current fan fiction maintains this focus on the fan, with guest appearances by the canon characters. Where pastiche celebrates the original work, fan fiction is narcissistic, fans celebrating themselves and using the trappings of the original to do it.
Not all derivative work is fan fiction, even if written by fans. Some few attempt pastiche and homage. But the great bulk is wish fulfillment and the celebration of fans, fan-inspired in-jokes, fan theories, or more often, a single fan. I personally think fan fiction is an early stage in a writer's development--mapping closely to the "chuunibyou" self-importance of pre-teens. Most writers outgrow this phase, and most fans put aside this type of writing as they mature in favor of other fannish pursuits.
I've noticed that geek culture tends to celebrate itself over the works it "loves" (hence the rise of "toxic fandoms"). And when icons of geek culture get into creative industries, their works, even with the blessings of the original creators and license holders, never rise above fan fiction. J. J. Abrams has done this with Star Wars and Star Trek, with both now more about the winks to the fans than telling a proper story. Characters, setting, and mood are sacrificed for fanservice. It's the nerd bet of the hardcores all over again, with the creators betting on geeks instead of reaching out to the audience. And, rather than being panned, this narcissism is celebrated and passed on to the next generation of creatives. Thus the rise of stories where the only bit of wonder is not from exploring a strange and wonderful world, but the self.
Frankly, our creative classes need to grow up and get over themselves.