Because 'slightly more obvious' is an important difference. Because he is making a lot of money simply by stealing other people's ideas. Because those other people are not making any money from his theft of their ideas, as Bauke said.

It's one thing to say that there are only a handful of story themes in the world, but there's archetypes and then there's plagiarism. If someone lifts your story wholesale, that's essentially theft. Now, I'm not going to accuse the writer of Eragon outright of plagiarism, because I haven't read the book and can't cite plagiaristic passages. But I do think, even if he has changed the wording enough to avoid charges of plagiarism, this is still a barrier to Eragon being called good writing. That's not good writing, it's good research and good PR.