Because that's what they have??? Cat fangs aren't like a snakes. They CAN NOT retract them into the skull. The fangs are permanently in place so if they bite you, you're probably going to get a fang related wound. However, cats do know how hard they need to bite to break the skin so if your cat ever does with you, it's because it feels threatened or intruded upon. Seek professional behavioral advice if you have a cat that always makes you bleed.