(Hint: It's because I'm not a writer.)
Well, that's only part of it. I wish I were a writer, but my brain does not work like a writer's. I admire the details and the subtle touches writers put into their work to create more depth in their plots/characters/settings/etc., for example, but my brain can't imagine itself writing those things in and can only marvel when a writer includes something that perfectly provides more shading in unexpected ways.
No, my creativity lies in playing with other people's words, not my own. And that's what I've always wanted to do: to be able to roll around in words, admire them, and put them in perfect order so they mean what they say in the exact right way - which makes me sound like a playful, reverent, type A control freak, but honestly, that's not too far off the mark. Probably because I spent so much time reading and so much time in my education analyzing the work of writers and because I had grammar drilled into me in elementary school (which tends to be unusual for North American schools, but I'm SO grateful for it), I have a pretty good intuitive understanding of how stories work and an ear for what sounds good and what doesn't.
Also, I really love working with writers. I like getting to know them, understanding what makes them tick, and watching them create a whole world out of words. It's always fascinating to see how the writing process differs for everyone and how individuals put their stories together. From the author who can sit down and bang out a fully-formed chapter in an hour to another who methodically researches the precise measurements, colours, and layout of a room a character spends one scene in to create memorable word pictures, every author I've encountered is unique in their methods. Not only is it very cool to be part of their creation process, it's an honor I take seriously. They trust me enough to hand over their work to me to tear apart and build back up again, and that's a BIG deal. It's why I like to get well acquainted with my authors before we start editing: so they can get to know me as much as I get to know them and they can feel that they're placing their trust in someone who gets them and their story.
Lastly: y'know, I think I'm pretty good at this editing thing. And I love it. And it's FUN. All that makes it the best job in the world for me.