Because monkeys and us shared a common primate ancestor and the resulting lines from that didn't die out.
All canines share a common ancestor, yet we have foxes, jackals, dogs, wolves, etc. Some lines died out like the dire wolf. The African Wild Dog is in the process of going extinct.
All felines share a common ancestor -- house cats, lions, tigers, cheetahs, servals, caracals, etc. Some lines did die out like the sabertooth, the cave lion, the American cheetah, etc.