Back to your point. Even if the supply is controlled by a monopoly that does not mean it can control demand. And there are hundreds of independent producers of oil with thousands of small wells throughout the country. These are not always in operation depending on whether the price is high enough to justify pumping.
In any case the barriers to entry posed by governmental regulation work to the benefit of the larger companies.
However we are not speaking of widgets, we are speaking of the life blood of the world, in particular the United States.
Supply and demand keeps the widget makers in line. However oil affects everyone, immediately.
This fact allows a handful of people around the world to manipulate the market, thus supply and demand is skewed in their favor.