Well one reason could be that it's cheaper than producing 7 billion robots. You know economic reasons. Not because they can't do it, it's just not cost effectiv

Well one reason could be that it's cheaper than producing 7 billion robots. You know economic reasons. Not because they can't do it, it's just not cost effective. It's much cheaper going to a planet and taking all of its population to work your asteroid mines in that solar system. Often we do things not because we are not capable of alternatives, but it's cheaper. So would the aliens. Or it could be as trivial as they enjoy eating meat, and this is a a good meat planet. It's cheaper than farming. You just go from planet to planet for meat. Replenish your world ship's freezers by harvesting the planet. Possibilities are infinite, or at least similar to the size of the universe itself.