this is a great point. fanfic is playing in someone else's sandbox for the love the characters/world. all of the heavy lifting is done already ie: characters, world-building, established dynamics and histories, etc. aspects can be changed to suit whatever the author wants to do but it's still playing with dolls. it's a great place to have fun but when fanworks are adapted into original stories, the stories often fall apart.
fanfic is a great place to hone skills as a writer and learn how to craft a narrative but taking a story that's supported by an existing IP, removing that structure then replacing it with something new rarely works. it's working backwards, forcing a square peg into a round hole, removing a solid framework with popsicle sticks. the structure is flimsy and the holes are glaring. putting the cart before the horse, etc etc.
i don't say this to disparage fanfic writing (i've written a few of my own) but it doesn't lend itself to original world-building. even if the author created many new aspects to the existing IP, the foundation isn't theirs and breaking it down to build something original is extremely challenging. more often than not, it feels like a copy of a copy of a copy, diluted to nothing but tropes because that's all that's left.