One of the reasons that many developing countries such as Mexico have a hard time developing a robust economy and a strong middle class is precisely because their wealth is concentrated in an oligopoly made up of a handful of powerful families who wield vast political and economic power. You'll see this all across Latin America, and in most other parts of the world.

Also, as you can see from the Russian billionaires on the list, many influential people have made vast fortunes when state-owned monopolies were "privatized" and became privately-owned monopolies.