I beleive it\'s because aizen made them with an abnormally strong skin, almost to the point of armour, as it\'s been seen,

SPOILER (click to view)

if the shinigami has enough reiatsu going into the attack(eg. bankai, or when they are putting enough into their normal attacks...) they can in fact cut the arrancar, tousen was able to cut grimmjaw, and ichigo was able to do a little damage, but it\'s a question of paower levels...