This always raises the question, why don't they just make the entire plane a black box so it survives a disaster?

Um, because the black box doesn't even always survive. People have this image that the black box is made out of some magical indestructible material that will survive a serious catestrophic event like a plane crash. I seem to recal that a lot of the time the black box is dead as well, or at least not in great shape.