Oil is used in lots of plastics, which are pretty essential structural materials these days. But we still burn it for fuel. And iron is far more abundant. Also

Oil is used in lots of plastics, which are pretty essential structural materials these days. But we still burn it for fuel. And iron is far more abundant. Also, I'm no chemist but I imagine it's a lot easier to strip an oxygen atom or two off of an iron oxide molecule than to string together a bunch of water and atmospheric CO2 molecules into a complex hydrocarbon. So we can convert the combustion byproducts back into fuel much more easily with iron.