Who's to say they haven't?
Ignoring the before mentioned reasons of Apple not caring about what people think, us≠ the majority of people who will buy the product, and Apple already pegging down the design years in advance, there are other, equally important reasons...
If 2-3 months were enough time for Apple to do a full redesign if needed, it'd also be enough time for any other company to create and market that product as their own.... after all, Apple's denial of it being theirs would make it fair game, right?
If they were to add a slight error or mistake in it's design, it wouldn't be an accurate presentation so what people thought of it wouldn't matter... unless it was a minor cosmetic error and not a mistake or change in it's design - eg. mirroring the logo so the bite is on the wrong side.