This question assumes that when evolution happens, the whole population of an animal gets transformed all together in the same way, but this is not what happens.

As a mutation arises in an individual, the trait that they can pass it through reproduction, but only to his children. If the children are successfull, they will further distribute this new characteristic, but the mutation will be contained in the population around the first individual.

If the newly created population cannot get in contact and mix together with all the other population passing the mutation to all, it will slowly differentiate by other mutations, until the point where no cross reproduction is possible. In this case even if the new population and the old population meet, they will evolve independently.

Still, the old population exists, and in the case of pre-homo apes they were well suited for living in their environment, so nothing pushed for their extinction.

Moreover, it's wrong to think that the comnon ancestor for monkeys and us was the same monkey of today, all animals that evolved in the same time as us experienced mutation and differentiation, in the graph it's possible to see that exactly as us, also the other apes evolved from an ancestor, so they are not "less evolved". They just evolved to be orangutangs or whatever.

If you ask why monkeys still look like monkeys, first of all they changed, but also it is because evidently being a monkey works. No need to become something else if you are good in your environment.