It's not less work to write in a world with existing characters than to make up new ones. There's a potential argument that it's harder--you have to match preconceived notions of how the characters will act--but I think that balances out with having existing traits to draw on. A believable "what happens next" story takes just as much creativity and literary skill as a "here's what happened to people you've never heard of" story.
Publishers don't have a habit of paying authors less if they're working with public domain texts as a base. Arthur Laurents didn't have to pay royalties to any Shakespeare Foundation for "West Side Story."
All Creative Work Is Derivative... we're all working with ideas created and developed by other people. It's not inherently less creative to use someone else's names or places than to use someone else's tropes, themes, and plotline--and those are free to borrow.
I'm not saying we should abolish copyright, or that all derivatives should be legal without permission, but that they're not innately less artistic than work where the source material is more blurry. The hundreds of fantasy authors who retold "Lord of the Rings" with different names and slight personality twists didn't get paid lower royalties than those who wrote other stories.