It could be that they have a different perspective on what constitutes good storytelling. They might view the 'Harry Potter' series as having too many elements that are not based in 'reality' (even though it's a fantasy) and thus consider it similar to the unrestrained and sometimes illogical nature of fanfiction. For instance, the way time - turners work in the story or the concept of the Deathly Hallows might seem too convoluted and 'nutty' to them compared to more straightforward narratives.
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.
Some people might be so used to more realistic or traditional literature that when they encounter the highly imaginative world of 'Harry Potter', they label it as 'nutty fanfiction'. The fact that there are things like magical creatures, flying broomsticks, and a hidden wizarding society seems too outlandish for them, so they compare it to the often over - the - top nature of fan - created stories.
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.
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 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.
Maybe because they are a big fan of the original 'Harry Potter' series and are tired of seeing really bad fanfiction. They want to emphasize that the real 'Harry Potter' is much better than those poorly - made versions.