Sharks did change. Do you know how many different kinds of small and big sharks exists?

But when a shark is a perfect animal then it is very likely that every new mutation does not make it better. It can (and did) change to a new shark, smaller, thinner or whatever. But to make it a better hunter than it already is? There is no chance. It can smell over long distances, it can feel electricity under water and it can kill prey very fast. It is a perfect animal and most mutation make it worse not better.

Because of this it did not improve over the years.

And concerning the need of evolvement: If a being like human beings do not die very quickly...if there is no chance that a mutated human has an advantage over other ones or can get more children ... then there is no need and even no chance for evolution because you have no advantage over your opponents. In this case evolution stopps. The new mutation will be a weird being and thats all.

You simply need competition for evolution.