
Same reason there are multiple dog breeds? one group of monkeys with certain traits changed over the course of generations a separate group of monkeys got different traits over the same time period and didn’t interbreed with the first group, so they diverged and none of the monkeys are exactly like their common ancestor.
Dogs and wolves is actually a fantastic example. Both modern dogs and modern grey wolves descend from the same ancestral population. Modern dogs don’t come from modern wolves, they are both descended from this ancestral group. That ancestor would have been called a wolf. Therefore dogs are technically still wolves, but they’re so different we don’t normally call them wolves.
It’s the same with us. Apes are divergent branch of monkeys that are so unusual we don’t usually call them monkeys. And we are a divergent group of apes that are so unusual we don’t usually call us apes. And you can look at the genetics to make out the branches. Our closest relatives are chimpanzees/bonobos. Next closest is the gorilla branch, then the orangutan branch, then the gibbon branch, then the old world monkey branch, then the new world monkey branch, and then tarsiers, and then lemurs.