And yet they're the form of government every other form immediately starts to become.
Dictatorship or any similar government where power lies in one man? Dictators need assistants. And any single assistant who was significantly more powerful than the others would become a threat to the dictator, so the assistants all end up as men of roughly the same influence. The dictator then dies, and the assistants split his power between them. You now have an oligarchy.
Democracy or any similar government in which power lies in the masses? Not everyone can participate equally in the decision making, so you must have delegates. These delegates will quickly centralize the political power in themselves, and be unopposed simply because nobody else wants to deal with the day-to-day crap that comes with running a country. Since it is in their own interests to do so, the delegates will expand their initial political power to the economic and social circles as quickly as possible, and you yet again have an oligarchy.
Even the supposed perfect form of communism, in which there were no issues with revolutionaries keeping power for themselves after the revolution, would become a political oligarchy, simply because some people have more talent at government than others do. From each man's ability to each man's need, as they say. Your oligarchy is those with the ability to lead.
So, since all forms of government (at least at the large scale) are just indirect ways of creating an oligarchy anyway, why bother designing anything but oligarchies?