I won't go on to ridicule this question although, in my humble opinion, it either shows a lack of understanding of the whole idea of evolution, or is being asked by someone looking to disprove evolution. And anyway, you can find the answer in a search engine so I shouldn't be answering it at all!
There are two ways of answering this question.
1. Shrug it off by stating Evolution is still a 'work-in-progress' and we don't really know (that answered that one!) or
2. To talk a little about evolution and try to answer by clearing up some common misconceptions.
I think the real question here is 'If something evolves, does the original which it evolves from die out' The answer to this is no.
The main thing here is that evolution does not mean the original dies out. The original only dies out if the conditions change enough for it to be unable to survive. Accompanying questions could be 'why are there no Neanderthal men still in existence but there are monkeys?' No Neanderthal men are in existence because the more evolved less - Neanderthal's killed all the more Neanderthal, less evolved's - hence their livable conditions expired. The rate of evolution dependant on conditions is also important to think about. Evolution is dependent on time as well. Vegetarian Monkeys and Apes didn't need to kill each other or anything else so they stayed placid and un evolved. The specific type of ape we evolved from was carnivorous and needed to evolve quickly to eat and keep up with the rest of its carnivorous friends or enemies!
I guess someone asking this question may be questioning evolution itself. Look around you. Evolution certainly exists - we see it everywhere. We know cells mutate and change - look at cancers. As animals, plants and us humans are all made of cells, we are all capable of mutating and changing our forms from one generation to the next. Over time, conditions and through necessity, our actions and our bodies will evolve to match the world around us. Note for example the increase in incidence's of short sightedness. This is a genetic defect and will gradually, from generation to generation become the predominant factor. As a result of corrective vision instruments, shortsighted people will not die out and will continue reproducing. Until we can alter the gene, more and more people will be born shortsighted.
I have drifted slightly from the question, so I'll summarise. Monkeys still exist because they had no enemies and no need to evolve, they therefore evolved slower than their apelike carnivorous counterparts - which no longer exist.
Another way to look at it is simply that humans followed a different evolutionary path from monkeys. We didn't evolve from monkeys - monkeys and humans evolved from the same ape like animal. We have since taken different paths, and both co-exist. There! 3 answers!!
Perhaps you biologists out there could provide a more detailed explanation than this lowly physicist- although I assume this question could quite easily form the basis of many a biology of studies of evolution thesis! Some other resources for your interest are as follows: Evolution questions from pharyngula Biology Online Why are there still monkeys