That's kinda like asking, "if evolution is true, why do I have cousins?" Think branching bush, not ladder. Common misconception. You really, really need to let go of this one if you think it is a clever disproof for Darwinian evolution. We and chimps share a common ancestor, just like you and your cousins do... If you go back far enough in your lineage (into prehistory, so you aren't likely to be able to do this...) you would find that you and Hitler share a common ancestor, as do you and Tiger Woods, or, in fact, you and Jesus Christ. Keep going back, and eventually... you and the chimp at the zoo.