They might have read a lot of Harry Potter fanfiction that has really over - the - top ideas. For instance, fanfictions that combine different magical creatures in very unusual ways or give characters powers that are completely unheard of in the original series. Also, some fanfictions may have really convoluted plotlines that seem to be all over the place, leading someone to think it's 'insane'.
It could be because of the sheer amount and diversity of fanfiction in the Harry Potter fandom. There are so many different takes on the story, from changing the relationships between characters to creating entirely new magical systems. Some of these can be so different from the source material that they seem 'insane' to those who are more used to the original Harry Potter story.
Perhaps they've come across some really strange fan - created stories. Some fanfictions can be very out there with their interpretations of the wizarding world.
Maybe they just don't understand it. Some people might be put off by the magic and supernatural elements at first glance and misjudge it as 'insane'. But in fact, once you start to explore the story, you'll find its depth.
Perhaps they don't fully understand the depth of the world - building. They might see all the magic and think it's just some random, crazy ideas thrown together like in a not - so - well - thought - out fanfic.
Perhaps they haven't fully understood the series. Some people might just skim through the books or movies and miss out on the depth. They could be focusing on just the surface elements like the magic spells and not realizing the complex relationships and themes beneath.
Maybe they want to emphasize the purity of the original 'Harry Potter' story. They don't want it to be confused with the many fan - made stories out there.
One reason could be that they feel the official Harry Potter story has a defined end, and the fighting in fanfiction is just over - done. They want to focus on the original tale's conclusion.
Maybe they just want to be controversial. Some people like to make extreme and untrue statements to get attention.
Maybe they just don't understand the real essence of Harry Potter. They might have only seen a small part of it or misinterpreted some of the more emotional or character - developing moments as signs of weakness, which is completely wrong.
One reason could be copyright. The creators and rights holders of Harry Potter want to protect their intellectual property. Fanfiction can sometimes blur the lines of ownership and might use elements of the story in ways that aren't authorized.
One reason could be that they are a huge fan of the original books and movies and think that fanfic often takes things in the wrong direction. They might feel that the creativity in fanfic goes against what they love about the original Harry Potter, so they reject it completely with that statement.
Someone might say it if the fanfic contains really bad writing. Like if the grammar and spelling are atrocious, it can be a turn - off. Also, if the story is just plain boring or the characters are not well - developed.