Maybe he lives in a world where fanfiction doesn't exist or he simply doesn't have access to it. For example, in the wizarding world of Harry Potter, their literature mainly focuses on magic textbooks, historical records of the wizarding world, and some classic wizarding novels. Fanfiction might be a completely foreign concept there.
There could be a more practical reason. Fanfiction might be considered a distraction from his real duties and studies in the wizarding world. Harry has a lot on his plate, like fighting against Voldemort, learning new spells, and dealing with the challenges at Hogwarts. Reading fanfiction could take away his focus from these important matters, so he is not allowed or simply doesn't have the opportunity to read it.
It could be that in this fanfiction, Harry was raised in an environment where reading wasn't emphasized or accessible. Or perhaps he had a traumatic experience that made it difficult for him to learn to read.
Perhaps there simply are no fanfiction works available in the wizarding world. The infrastructure for creating and distributing fanfiction, such as the Internet or fan clubs, doesn't exist there. Harry is too busy with his own life at Hogwarts, dealing with classes, Quidditch, and the constant threat of the Dark Arts to even think about something that doesn't exist in his realm, like fanfiction.
Maybe it's because fanfiction exists in a written or digital form and doesn't have an audible component by default. Harry, like any normal person, can't just magically hear the words of fanfiction unless it's been read out loud to him or turned into an audiobook.
Maybe Neville is too busy with his studies at Hogwarts. He has a lot of magic to learn, like Herbology and Defense Against the Dark Arts. Fanfiction might not be on his radar as he's focused on real magical knowledge.
Inuyasha is a fictional character from a specific anime world. Fanfiction exists in our real world, outside of his fictional realm, so he has no access to it and can't read it.
Well, in the Naruto universe, there's no concept of fanfiction. Naruto is busy with his ninja world stuff like fighting bad guys, training, and protecting his village. Fanfiction is something created by fans in our real world, not part of his world. So, he simply can't access it because it doesn't exist in his realm.
Think about it. Naruto's world has its own set of literature and stories, like the tales of the Sage of Six Paths. Fanfiction is something created by fans outside of his universe. He doesn't have the ability to access our world's internet or libraries where fanfiction exists. Also, his upbringing and the nature of his society don't prepare him for such a concept. He's more into learning ninja techniques and fulfilling his duties as a shinobi.