Biodiesel is made from vegetable oils, which are hydrocarbons (or are composed mostly of hydrocarbon molecules). The veggie oil doesn't need to be turned into hydrocarbons because it already is hydrocarbons.
My question is about the carbohydrates in plants (sugars, starch, cellulose), not the oils in plants. I was thinking that carbohydrates should be easily be turned into hydrocarbons given that both were just combinations of carbon and hydrogen. But as it turns out, carbohydrates are fundamentally different from hydrocarbons because they have oxygen as part of their basic structure (there are probably other differences as well, but that was the main one I picked up on as I Googled around). This (apparently) is why plant carbs aren't turned into gasoline or diesel, but alcohol.