Sharks have to keep moving, or they die!
Not all sharks have to move. Sharks do, admittedly, need to keep water moving over their gills in order to breathe. But many species manage to do this while lying on the seabed and moving their gills to create a pumping effect. Others, however, like the great white shark, possess no muscles in their gills, cannot pump and do have to keep moving.
No wonder the great white shark is so darn hungry.