[P-40]

Since plants store energy as carbohydrates, why isn't it a simple process to convert them into hydrocarbons?

You can convert to hydrocarbons for biodiesel. I think for gasoline you have to create the alcohol because that is the only way it will work.

This is from biodiesel.com

Technically, biodiesel is Vegetable Oil Methyl Ester. It is formed by removing the glycerol molecule from vegetable oil in the form of glycerin (soap). Once the glycerin is removed from the oil, the remaining molecules are, to a diesel engine, similar to petroleum diesel fuel. There are some notable differences. The biodiesel molecules are very simple hydrocarbon chains, containing no sulfur, ring molecules or aromatics associated with fossil fuels. Biodiesel is made up of almost 10% oxygen, making it a naturally "oxygenated" fuel.

Biodiesel is the only product known to the State of California to not cause cancer. :)

[Yardstick]

I did a little Googling to try to answer my own question and the nub of it is this: unlike hydrocarbons, which consist only of hydrogen and carbon, carbohydrates also contain oxygen. That's the key. They're partially oxidized, which means they have less chemical energy -- and it also means that you converting them into a hydrocarbon is not a trivial thing.