User blog comment:The Milkman/Everybody Shut Up./@comment-4553238-20120424234358/@comment-1341687-20120425063330

They keep it in Earth's orbit to better access the human population. Ferrying them around would be a waste of time and effort. As well as there is no where they could take it that everyone else couldn't get to without causing a lot of hassle and wasting time. Better to concentrate defense at the harvest grounds.