Matt Taibbi in his book Griftopia (last chapter) offers this explanation: World flow of oil money may be huge, but it is dwarfed by retirement savings --- controlled by Wall Street. Farmers and oil men hate price fluctuations, while Wall Street has no money to make without price fluctuations. In 1932 USA legislated against speculators, but they buy congressmen to re-enable their chaos. The 1932 law was 37 pages long. Its recent replacement is 848 pages long. (Besides price fluctuation, speculators need information asymmetry, easily created by laws that are too long to read. Secrecy == corruption.)