The aliens are very disappointed in us. If we wreck all the prettiness of the planet and use up all its treasures, what’ll be left for them to conquer and acquire?
Of course, this might seem like motivation for us pusillanimous pigs to keep trashing the earth–eliminate everything desirable and we’ll never be attacked by aliens who want it.
Except that even in our dullest-witted science fiction, we tend to acknowledge that alien races not only might be light years smarter and more advanced than we are, they probably also have different needs and desires than ours.
So they might just be sad because we haven’t managed to wipe ourselves out quite yet, meaning that they’ll still have cleanup to do when they arrive.
Marauding and usurpation are just as much hassle as ever. Unless we perfect self-annihilation as quickly as our present rate would seem to presage.