So I suppose you figured that Mozart's "Twelve Variations on Vous dirai-je, Maman" was a big let down
after bar 24. (It's a masterwork of theme and variation, the theme being the tune of old French folk song, which you'd now recognize as "Twinkle, Twinkle Little Star".) And I suppose there is no point in ever going to Morimoto's for sushi. After all, I'm sure that it's no different from the sushi available at the supermarket. And it was pretty pointless to make The Wizard of Oz in 1939, given that it had just been made in 1925.
I'm being facetious, of course. My point is that some of the very best things I've enjoyed were cases where someone added their own originality and creative vision to something preexisting. The usage of a preexisting universe - especially with regard to fantasy, science fiction, and related genres - allows the creator to focus on building up above the surface level, instead of having to waste so much of the readers' (viewers') time building the ground level itself.