I’m an artist and a writer, and I do actually know a few things about copyright law, and there are some misconceptions/lack of information here. I get it, if you’re not an artist yourself, you probably don’t know what it’s like. So maybe I can set a few things straight.
There’s a misconception here about just how legal/okay monetising fanart is. Just because someone puts their fanart up on Redbubble doesn’t mean it’s legal. In fact, fanart gets taken down on Redbubble all the time because the copyright holders issue a takedown notice. And if you get too many take-down notices, RB will close your account, because you’re breaking their TOS. Some creators are okay with fanart merch; for a small-ish TV show or book, fan merch might be helpful, since it increases fan investment and helps promote the thing without them having to invest a single dollar, and it’s not competing with their own merch because they’re not selling merch. It may not be technically legal, but the creators simply won’t do anything about it. But try putting up Marvel fanart or Disney princess fanart, and see how long it takes before Disney sends you a cease-and-desist. I mean, they went after people crocheting Baby Yodas on Etsy.
The whole “fair use” thing is really a philosophical debate. In reality, if the copyright holder issues a takedown notice, the thing gets taken down. Yes, you can file a counter-notice, but that constitutes a legal agreement to go to court over it. And realistically, there’s no way any of us are going to win against Disney’s lawyers, fair use or not. You do not want to sign up for that, believe me. Much better to just let it go.
What saves most fanartists isn’t the fact that it’s legal, it’s the fact that a cease-and-desist is usually a copyright holder’s best approach. If you issue a CAD, you can’t then sue someone if they comply, unless they file a counter-notice. It’s the easiest and cheapest approach for Disney and in 99% of cases it makes the most sense, and luckily, is also the one that lets the fanartist off the hook with nothing more than having to take the work down. I don’t know how likely it is that this would also be the approach for monetised fanfiction.
Another important thing: fanfic is a different medium. Gifs and fan videos are also examples of fanworks that you can’t monetise. It’s not just fanfic. It’s partly down to the medium and the fact that you can’t sell merch with anything except fanfic. Nobody pays just to look at fanart, trust me.
And a huge factor is that most fanfic is based on stories - films, books, etc. Fanfic is therefore automatically less transformative and a more direct rival to the source, especially if that source is or includes books.
I mean, think about it like this. You write a fanfic. Someone draws an illustration for it. Most writers would think that’s awesome, right? Now imagine someone rewrites your fic with a different ending, or takes your premise and all the little details you came up with and basically steals them for their own fic. Still awesome? Because most fic writers consider that plagiarism, and get very annoyed.
So authors, especially, are often cool with fanart but not with fanfic. Because fanfic is essentially taking the thing you wrote, and rewriting it. Even fanfic writers don’t like it when someone takes the thing they wrote and copy or rewrite it, so maybe you can understand why original creators don’t like it either? Now add money into the equation, and imagine that you put all this time and effort into creating characters, getting published, etc - and someone else just copies your work and sells that. Would you be okay with that?
The exact same thing happens with art, by the way. It’s just that most fanart is also based on stories, not paintings or photos. But if you copy an artist’s painting and call it “fanart” and sell it, I promise you, they’re going to be pissed off. You’re much more likely to get away with using it as inspiration for a story. In fact, I think writers tend to get away with this more, if anything - you can write a story inspired by a photo you saw, and the photographer will likely have NO chance of proving that it’s essentially fanfic of their work. If you wanted to write and sell fanfic based on some artist’s paintings, I’m pretty sure the tables would be turned and you would get away with it while a fanartist trying to sell copies or re-interpretations would not.
Also: there are creators and celebrities who are cool with fanfic and have explicitly support it, and there are instances where fanart has been laughed at (I distinctly remember them showing pinup art to Hayden Christensen on some talk show to embarass him and laugh at it; same with Chris Hemsworth).
And finally, you can sell fanfic if you change the names and details and make it an original novel. Sure, there might be people who sneer at it, but trust me, there’s people who sneer at artists for all kinds of reasons, too. (Did you know that digital art isn’t “real” art? And that charging money for your art devalues it and artists should starve for their art if they really mean it? And that if I put my art on the internet, it’s free game for anyone to take and do whatever they want with it? And that fanart isn’t “real” art? The list goes on.)
I’ve got firsthand experience with both sides here. So I can tell you that if you think selling fanart is just always allowed, and artists don’t have to deal with any kind of backlash or people looking down on them… you don’t have the full picture. It’s not nearly as simple or as one-sided as it might seem.