Lets assume that fact that we descended from monkeys are true. Why there are still monkeys around you ask. Well first of all, i guess that not all monkeys would be able to evolve. Some of them would stay monkeys, and that explain your question. And it would take a lot of time for evolution, from one specie to another.
In the inland sea in Japan, there were crabs who had curious markings on their backs that resemble a samurai's face. All the fisherman in the area thought these were sacred and when caught them, they throw them back in the sea. As time went on, they soon discovered that all there was an over population of these crabs and all other crabs became scarce. If you were a crab with these markings, you had a much better chance of survival. It's called artificial selection. This occurs all the time in nature. The strong survive. The weak die off. Evolution is not a theory it's a fact!