There was some era where teachers were telling students that the main thing was to get their stuff down on paper and they could work on format later. Which lead to a truly horrendous period where all the newbie writers were releasing stories full of mis-spellings, no punctuation or paragraphs, constantly shifting verbs and POVs, lack of plot, horrible characterization, etc etc etc. Think of every single writing crime you know of, and these stories were full of them.
And you'd very gently write them and suggest maybe punctuation would make it easier to read, or one sentence shouldn't take up two pages, or actually naming the character who was doing or saying something instead of just expecting the reader to pull it telepathically outv of the ether. And they'd get all defensive, "Well it's MY story and everyone else has liked it and doing all that stuff just slows me down and the important thing is just to get the thing down". You'd suggest maybe they could use a beta reader, but no that was too much work as well.
I had to give up on entire (small) fandoms because the writers simply. Could. Not. Write. I'm really glad that era seems to be over.
One thing I love about fanfic is that it's a shortcut to storytelling. So, like, you pick up a new novel, not part of a series or anything, and you start to read. And you have to map out the entire milieu in your brain: these are the main characters, these are their relationships, this is their backstory, these are the things you need to remember about the location or politics or whatever else there is.
You spend all this time absorbing all this information and learning to really care about the characters and following events as they unfold and then -- it just ends. And then you pick up another story and you have to do the whole thing all over again.
The nice thing about fanfic is that you can skip all the universe-setting stuff: you can (generally) assume that a reader will know the characters, relationships, back stories, etc etc etc. You can literally start a story with the line "Dudley chased Harry down the street" and not have to explain who Harry is, who Dudley is, why Dudley is chasing Harry, why Harry can't avoid or hide from Dudley's attention, why Harry's best option is to run, vaguely how old Harry and Dudley are, why Harry can't get someone to help him - hell, you even know it's summertime. Fanfic is a shortcut to storytelling, enabling both the reader and the writer to get to the meat of the story without (necessarily) needing to faff about explaining and trying to remember all sorts of details.