Honestly why bother with invading and mining the Earth?
Isn't the asteroid belt filled with minerals that could be gotten at with no hostile forces to deal with or even planets like Mars, or the Moon for that matter? Why bother with Earth?
Aliens are unpredictable & tricky. You can never figure out their motives or what they might do next. And forget trying to have an intelligent conversation with them. They come to invade our planet and don't even bother learning our languages.
~ the hardest thing in this world... is to live in it ~
Who's to say they aren't also mining the rest of our solar system? We just get to see the Earth portion of their mining expedition because it would make for a much less interesting time to watch them stripping uninhabited planets and space debris instead.
And who knows, they seem pretty keen to disintegrate people and other animals alike so maybe our energy acts as food or powers them or something?