I hear this question frequently from people (especially creationists). The answer is actually pretty simple. A better question would be, "if humans evolved from fish, why are there still fish?" Fish don't have lungs, can't walk on land, can't fly and are slimy. Surely us modern mammals like humans, bats, and tigers are so much more advanced! Why haven't fish gone extinct?
The easy answer? Fish are still around for the same reason bicycles are still popular years after the invention of cars and rocket ships: they are configured for different environments. Sure, fish can't do some of the great things that humans and birds do, but the job they do (swimming), they do very well. A bicycle is no moon rocket, but it's a much better vehicle for a quick run through the city park--or crowded streets. Heck, even after all these years of bicycles and cars, policemen still oftentimes ride horses!
Creationists make hay over the occasional discovery of a "living fossil." Once in a while we find a living animal that we previously thought was extinct. Sometimes the living animal is very similar to million-year-old ancestors. But again, in the case of a fish (like the famous coelacanth), evolution long ago arrived at the best shape for living in the oceans. Why would we expect it to look different?
Explaining the persistence of perfectly functional animals like fish or apes is not at all difficult. Evolution adapts animals to their environments. As long as those environments are still around (oceans, deserts, forests, etc.), animals that are well-adapted to them have a shot at being around for a long time as well.