"Why are there still monkeys?"

This is the stupidest argument I have seen. In general. And like most arguments, it requires a simple explanation.

We did not evolve from modern monkeys. We evolved from primitive monkeys, or rather, primitive apes. This species eventually "took part" (for lack of better phrasing) in speciation.

Now, speciation is where an ancestor species branches off into two different daughter species. The primary way of this is either allopatric or quantum speciation. Allopatric speciation is where a part of the geographic population of an ancestor species is isolated and has to adapt to a new enviroment (natural selection). This creates a daughter species (possibly multiple) to the ancestor species.

Quantum speciation is the rapid evolution of daughter species. It works for punctual equilibrium (the rapid change of a species.) It basically works in the same way allopatric speciation does.

This goes for humans and modern monkeys. There was a speciation, in which modern monkeys adapted differently to the enviroment.

Also, you can turn the question on believers: If Adam was made from dust, why is there still dust?

Alas, another fallacious argument.