My guess? Energy usage. One reason why crude oil, a fuel source, is also such a good chemical feedstock is that it contains a lot of complex, high-energy hydro

My guess? Energy usage. One reason why crude oil, a fuel source, is also such a good chemical feedstock is that it contains a lot of complex, high-energy hydrocarbons. These are the kinds of complicated molecules that require a lot of energy input to form, and hence release lots of energy when burned. Reversing this process and turning simple carbohydrates plus carbon dioxide into complex carbohydrates is usually a highly endothermic reaction.