How Oligarchies Form
http://www.wisegeek.org/what-is-an-oligarchy.htm
Oligarchies are often controlled by a few powerful families whose children are raised and mentored to become inheritors of power, often at some sort of expense to those governed. In contrast with aristocracy — or government by the “best” — this power might not always be exercised openly, with some oligarchs preferring to remain “the power behind the throne,” exerting their control through economic means. Unlike plutocracy, oligarchy is not always a rule by wealth, either, because oligarchs can simply be a privileged cadre. It also has been suggested that most communist states fit the definition of oligarchies.
It may not always be “families” running the oligarchy but chosen small groups. Power in the hands of a select few, and kept that way.
Hey, if you and your pals owned the USA and a lot of the world, you wouldn’t want to give that up easily.