The Overlords are much more intelligent than the human race, able to easily comprehend things our brains would struggle with. And as we learn right away when their spaceships appear over Earth, they are much more advanced. The problem for the Overlords is that they understand that there is a sort of a leap in evolution possible for some intelligent species, such as humanity. We can evolve into a form that has little need for a physical body, and is a sort of energy. The Overlords can't, and this is their tragedy. They understand this higher state and serve it, but can never experience it. And the irony is that it is their much greater intellect that prevents them from entering this state.