My guess for why they don't have the buttons change function is because you wouldn't have a single clue which one did which if the phone/tablet were laying flat on a surface. You wouldn't have any idea what position the tablet was in the last time it was picked up, so you'd end up hitting the wrong button and get frustrated.

Best just leaving the buttons to do the same thing each time.

Of course, I have to bring it up: I'd be willing to bet that there's an Android application that would let you do this. I wouldn't ever want to install it, because I'd hate for my volume buttons to work this way, but those who would want it could have it.