Noise cancelling headphones work by playing the exact opposite sound wave, to cancel it out to nothing. This means that they can only cancel sounds that are as loud as the loudest sound they can play, anything louder they won't be able to cancel out fully. And because they don't want to get sued / fined for playing noise at ear-damaging levels, they cannot cancel out noises that are ear-damagingly loud. They can help a bit, because they do cover your ears and hence dull the sound a little, but they cannot substitute proper ear protection.