Consider how many books exist on how to care for trees. Each one of them has similar ideas, but the way those ideas are expressed differ. Copyright protects the content of the book; it doesn’t protect the ideas of how to care for trees.
I understand what you're saying but the way you're framing it isn't what I really have a problem with. I still don't agree with the idea that I can't make my own physical copies of Harry Potters books, identical word for word. I think people can choose to buy the physical books from the original publisher because they want to support them or like the idea that it's the "true" physical copy. And I'm going to push back on that a million times less than the concept of things like Moana comic books. But still, it's infringing copyright for me to make Moana comic books in my own home, in private, and never showing them to anyone. And that's ridiculous.
It’s very unlikely that she would (or even could) have devoted herself to writing fiction in her free time as a passion project without hope of monetary reward and without any way to live from her writing for the ten years it took to finish the Potter series.
And even if she had somehow managed, you’d never hear about it, because without publishers to act as gatekeepers it’d have been lost in the mountains of fanfic and whatever other slop amateur writers upload to the internet.