My theory is that the main alien fleet was invading Mars or some other planet. The aliens that showed up on Earth were the really incompetent and useless members of the fleet, and the commander saw an easy opportunity to get rid of them.
"Good luck invading that blue-green planet, Ugga-Thathlu!"
"Er...I heard that it's mostly covered in a liquid that kills us, that the inhabitants are mainly composed of that liquid and always have easy access to it, and that it falls from the sky in great amounts, regularly. Shouldn't we at least take some protective gear? Or weapons."
"Nonsense, you'll be fine!"
"Good luck invading that blue-green planet, Ugga-Thathlu!"
"Er...I heard that it's mostly covered in a liquid that kills us, that the inhabitants are mainly composed of that liquid and always have easy access to it, and that it falls from the sky in great amounts, regularly. Shouldn't we at least take some protective gear? Or weapons."
"Nonsense, you'll be fine!"