That's because, as a professional educator, I consider it my duty to squash any creativity I see in my students, to extinguish even the slightest spark of original thought, and to train hordes of mindless workers to parrot back what I've taught them.
Well, no, not really. All the best papers I've seen have been “creative” in some important sense. But when students ignore the assignment and give me something very different from what I ask for — a retelling of the novel from another character's point of view, an original poem in the form of the one we're studying, something like that — I'm in a tough position. That's not because I dislike students' taking the initiative and finding a new way to talk about a text, but because I give assignments with certain things in mind.
Perhaps you're endlessly inventive and witty, and can reel off one brilliant word after another. That's swell. If, however, an optometrist has asked you to read an eye-chart, your creativity doesn't tell him or her how well you can see. In the same way, a short story telling The Great Gatsby from Daisy's point of view, or a version of Romeo and Juliet where they live happily ever after, may be fun. They might show me you're a fine stylist, or even that you grasp the central concerns of the books. But they won't tell me whether you've mastered other skills of argumentation and organization that I'm paid to teach you.
So here's the deal: if you want to do something off the wall with your English paper, talk to your professor first. He or she may encourage you to go nuts and let your imagination run wild. But you might also be told to hold off, and to stick to the script. That's not out of any hatred for creativity, but out of a sense that there are times when it's best directed into certain channels.