they are made of different muscle fibers
very slow twitch since they are always used
and like serge nubret says, they NEVER burn out no matter how much you train them
and they don't get MUCH bigger than they already are

So you don't think ab muscles atrophy like every other muscle in your body?
Your arms, legs, chest, back, etc can all shrivel up and go away and you'll still have ripped abs?