Same reason God invented mosquitoes - because they were supposed to do something on this planet. Then sin messed things up and everything got a little out of kilter.
Did you hear about mosquitoes? Scientists say they were supposed to be pollinators. But one single defective gene turned them into blood suckers. They can still survive on flowers, you know, if there are no red-bloods around. They don't like it, but they can survive as pollinators.
Scientists haven't turned up any such information about roaches and rats yet, but now I'm curious what they were doing before the world was marred.