^^ Then why not make up a new idea?
Why doesn't a painter make up new people instead of painting portraits of real ones? Why didn't Shakespeare invent a totally new verse form instead of writing a whole bunch of sonnets? Why do so many works of music use the same rhythm and chord structures?
Every new idea builds on old ideas. That's how creativity works. Inspiration comes from experience. Structure comes from drawing on established elements and patterns. The structure you choose is just the substrate. Its familiarity is the anchor for the audience. Creativity is in what you build onto that framework, how you modify and augment it.
And it shouldn't even be necessary to defend this. Human beings have been retelling old stories for as long as storytelling has existed. Before there was widespread literacy, that was the only way to keep worthwhile stories alive. It's not unoriginal, it's not a "ripoff," and it's not something that any creator needs to justify (so long as they have the legal right to adapt the property in question). Originality isn't in where your ideas come from -- it's in where you take them next.