So why do you think there are still Monkeys, why would the process that can accidentally endow significant design improvements in a single individual, withhold this obvious advantage from monkeys? for millions of years?
I want you to image something for a moment. Imagine you have two groups of apes. Not unheard of, right? Apes travel in groups, they don't travel in one giant ape group.
So take Ape Group 1, and take Ape Group 2. They start in a similar area, but separate, maybe do to weather, maybe just following the food source around... Ape Group 1 more or less travels around in areas that are pretty similar to the original area. So, they have kids, the kids look like normal, and for the most part none of them die. Ape Group 2 go North into some mountainous area and stumble upon an elevated region where it's 40 degrees. Generally, the kids Ape Group 2 is having is having a harder time surviving the cold. However, one of the ape's is born with a random mutation where their fur is slightly thicker. The baby lives, and goes on to reproduce a bunch, and all of its kids have slightly thicker coats, and so none of them die in the cold. However, all the other kids in Ape Group 2 are dying. So any ape in Ape Group 2 with that gene for thicker coats, is going to reproduce more effectively, meaning the gene for the thicker coat is going to become more abundant then the original gene. However, in Ape Group 1, that same mutation for a thicker coat would a problem, because it's not actually colder there.
So you have Ape Group 2 with a different gene frequency than Ape Group 1, even though they all the same ancestor.
It's no different for brains... monkeys have different sized brains. The brain slowly grew bigger, allowing beings to get slowly smarter over time...