because they jump out of the water and try to bite your face.
i'm not worried about the underwater sharks - the bulls, tigers, lemons, sandsharks, even those silly looking hammerheads - you almost never see them jump up out of the water. they cruise by. how are they gonna bite your face if they just swim around in the water?
but a spinner shark will jump up and bite your face!
also the spotted eagle rays are easy to see when they jump out of the water and try to stab you in the chest with their bayonet tails as they steal all your surf wax.