Short answer -- plants produce carbohydrates, so that's what animals use.

Corollary question -- why don't plants produce hydrocarbons instead?

I think the answer is, plant photosynthesis (well, back then it was cyanobacteria photosynthesis) developed in the absence of free oxygen. Carbohydrates break down into water and CO2 with net release of energy without oxygen -- it's called anaerobic respiration. Hence carbohydrates were useful to plants as energy storage, and to animal which developed to eat plants. Hydrocarbons are at the bottom of energy well -- they do not exothermicaly break down into anything, so they were of no use to the plants.

__________________
Fiction has to be plausible. Reality is under no such constraint.