I have no recollection of reading them but remember being crazy about the books in the late '90s and early 2000s when they first came out. When each one came out I'd read them pretty much in one sitting.

They felt big and grown up because they were so big, but the story was pretty basic and easy to read. It created an exciting fantasy. They're not particularly brilliant as pieces of writing, but there wasn't really anything else at the time and it was a very rare instance of a book becoming a huge cultural phenomenon.

I stopped reading them after 4-5 books when there was a long gap between books and I grew out of them.

The enduring popularity is similar to Star Wars. The original Star Wars films are objectively terrible on almost every level, but it was unlike anything else when it came out. Young people at the time loved them, grew up with them, and then introduced it to their kids and so on. All thr while new Star Wars stuff would come out. I got into Star Wars when the original trilogy got re-released ahead of the prequels coming out. Now I'll still watch Star Wars stuff out of nostalgia more than anything.

Harry Potter is the same, but they're also films. And there have been spin-offs and additional media. Parents introduce kids to the books, kids too young for the books would be the right age when the films came out, etc etc.

Quality is very rarely the cause of a cultural phenomenon. It's a case of timing, capturing imagination, and marketing.

JK Rowling didn't write a great book, she created a fantasy world with very wide appeal.