"Hey! Here's a planet full of mineral deposits and other resources. It's even got these violent monkeys we can get addicted to shiny things and teach how to use basic tools. Now all we have to do is wait a couple hundred thousand years, they'll have killed themselves off and mined everything up out of the ground into nice, neat, easy to recycle piles for us."
Of course, if there were older civilizations, there's no reason to suspect we wouldn't see some kind of evidence of that. What is stopping a civilization from becoming advanced that far in the past? I honestly think we should be seeing ancient dyson swarms around distant stars.