 Why is the Earth worse than the aliens imagine?
Earth was so far away that it wasn't visible. But this vast distance did not mean that isolation could endure forever. Instruments within the ship intercepted radio broadcasts and, within the hour, early TV signals. The history of the planet was tabulated as facts became available. A meteor shower could pass over and their clumsy instruments could interpret it as an all-out enemy attack." "And how long can Earth last? They went much closer to Earth, not intending to commit themselves. For a day they circled the planet, avoiding radar detection, which for them was not difficult, testing, and sampling. Atmospheric analysis showed that as far away as we were."
 The Earth has missiles and is close to space travel.
 The Earth has missiles and is close to space travel.  The humans are rough and desperate.  A meteor shower could be interpreted as an enemy attack by the humans' clumsy instruments.  The humans don't like aliens.