This is the reason that no shark bones have been found. The cartilage plays an important role in a sharks survival and buoyancy. Sharks have no gill muscles so they must continue to swim in order to breathe. One exception is the Nurse shark. This shark has gill muscles and spends most of it's time on the bottom of the ocean waiting for food to scuttle by.
Sharks also hunt.
The cartilage helps relieve the stress of the water resistance as the shark swims. The sharks skeleton is hollow on the inside. Each bone is filled with a mixture of air and fluid. The shark can regulate the amount of air in these bones by secreting a fluid that makes the body of the fish heavier. In turn, the shark slowly descends into the depths of the ocean.