There are two rather serious faults with this question, each of which betray a deep misunderstanding of what evolution is and what its claims are.
The first issue is rather obvious… Even if we were descended from monkeys, why wouldn’t there still be monkeys? Evolution takes many branches at many rates. To ask “why are there still monkeys” is like saying “North Americans are descended from Europeans, so why are there still Europeans?” That one species or race evolves from another does not in any way require the extinction of the predecessor.
The second problem is this: no scientist in the history of discovery has ever said that human beings are descended from monkeys. No scientist has ever said we are descended from apes either. In fact, no scientist has ever said that we are descended from any species alive today. They’ve never said it. Darwin never said it.
What Darwin did say, what is in his book “the origin of species,” and what is commonly accepted today, is that we share a common ancestor with the ape. we did not come from apes, and they did not come from us, but rather we both came from a species that died out a very long, long, long time ago. In the evolution of species, apes are not our parents, they are our cousins. Far back enough down the line, our ancestors and their ancestors came from the same place.
That may seem like a fine point, a subtle distinction, but not only is it not a trivial detail, but failure to understand this point has resulted in a great deal of wasted time as people attack evolution for things no evolutionary biologist has ever even suggested. We did not come from any species alive on this planet today.