Why the boneyard?

Why are all those planes, which at leat to me look like they'll never fly again,
sent out into the desert to sit instead of just being hacked up for scrap metal?
I hate to see that happen to any old airplane that's given its best, but it
seems to be a big expense to store them like that. Or do they really consider
them "in reserve" in case they're ever needed again? I can't imagine what it
would cost to try to get them airworthy.