Humans did not evolve from monkeys; humans and monkeys both evolved from an extinct common ancestor.
Evolution is tiny little changes over millions and millions of years. Each little change is a mutation.
Maybe a mammal was born that was taller and stronger, so this mammal would live longer and have more children, all of which could be taller and stronger.
Then one of that mammal’s great great grandchildren would have a bigger brain, and live longer, and pass its bigger brain onto its children. Then this creature eventually evolved into us humans.
Meanwhile there was another set of mammals that mutated to be better at jumping through trees, or using tools. These then evolved into the monkeys we all know!
So monkeys are here because they evolved one way and we evolved into another.