The Mi-go lack most of what might be expected from interstellar travelers. They have no spaceships or spacesuits, nor do they use buildings or vehicles of any kind. Mi-go tools are custom grown appendages, ready to be re-absorbed and re-made at will. Their bodies can withstand the interstellar depths, and they grow massive, wing-like solar sails for propulsion. The Mi-go long ago became masters of practical biology, and have never stopped refining themselves.
The Mi-go focus on biology is the key to their interest in Earth. They are not seeking water, metals, or radioactive material, they are seeking life. Humans are of interest, but they are also a growing threat to Mi-go activity. The attempted American moon landing, although a tragic failure, was a turning point for the Mi-go. They could no longer remain hidden simply by keeping to the impenetrable mountains. The human problem would have to be solved.
Above every other goal in this war, above even victory, is the Mi-go determination to remain hidden. Second, again above victory, is the preservation of a diverse biosphere for future exploitation. The Mi-go wish to take humanity to the edge of extinction with a soft touch.
The Mi-go attack, like everything else of theirs, is biological. The introduction of a carefully engineered mutagen to Earth is turning animals into monsters. Every week there is a new disease, someone spots a strange new beast, or a strange new carcass washes up on the beach. From the bacteria up, Earth is swiftly becoming an alien planet.