How can any government derive their just powers by anything other than the consent of the governed?

Theoretically, yes. However, the truth is that the average citizen simply wants to live life, get their work done and spend some time enjoying themselves. You can't blame people for that. For the people to constantly be in control of their government there would have to be a never-ending revolution. That would call for leaders and plans and organization and, surprise, now we just have another government to watch out for.