Well, just to be straight, I do recognize that the view from very far above the Earth exhibit the appearance of curvature. But not from most commercial airliners (which I assumed was what you were talking about). If you thought you saw curvature, it is likely because you thought you should have seen curvature; it's not an optical illusion so much as it is your subconscious lying to you. You probably heard you could see the curvature of the Earth from a plane, from someone who didn't know what they were talking about, or else otherwise thought you should since you're so high up, so your brain told you you could even though you couldn't.