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.
I strongly agree with your brilliant idea. If we really are from monkeys, then why other monkeys are not evolving to human? But there are also some evidences that would somehow show us that human structures yesterday are different from us today. But these are still not enough to prove that the theory of evolution is true.