Their skeleton is made of cartilage, the same substance that your nose is made of. It's more flexible and lighter than bones are. This makes it more durable, and light, which saves them energy. This is particularly useful because sharks have to constantly swim in order to not drown. Sharks also lack a rib cage, which means that they can be crushed by their own weight on land!
Now you know. Next time a shark's about to eat you, you can at least be consoled that it's a spineless animal. Or you could get Orcas to help you