Human Civilization has been around for a mere blip compared to the eons that the universe and its trillions of stars have been developing. If they're out there, they'll likely to be far more advanced and would look at us as we watch an ant farm. Awwww, how cute; look at them working. (Of course, there are also just as many who'd be far less advanced.) But to assume that Earth would be an ideal planet for invaders would also assume that the Earth would be an ideal environment for ETs. It surely might not be.