I think it is because the supply of oil is ultimately limited. Current estimates are that we have oil left for about 47 years. Nearly all of that is already own

I think it is because the supply of oil is ultimately limited. Current estimates are that we have oil left for about 47 years. Nearly all of that is already owned by some oil company, but not counted as an asset. Therefore, oil companies can profit immensely if the price of oil increases.