Because it is artificially controlled. While the Oil companies would like you to believe it's supply and demand... that is only partially the truth.
The US Dollar is the basis of trade. US Dollars are FIAT money... printed with zero backing. The value is only whatever is perceived by the public. The Government says it has value so it does... until people stop believing in it.
The reality is the oil prices will always be manipulated for the most profit for the people who control the money supply... the shareholders.
People think that inflation is "rising prices"... but it's actually not. Inflation is the expansion of the money supply. Rising prices are a symptom of the expansion. In simpler terms inflation is the devaluation of the dollar because there is additional FIAT money flooding the market.