Simple hydrocarbons like methane and ethanol are produced by many organisms as byproducts or waste products.
Methanogen bacteria were probably the most common form of life at one time on our world (I think) so there were plenty of producers of these compounds; but as methane and ethanol are volatile perhaps there were simply no good ways for a life form to develop which could collect and use them.
A hydrocarbon metabolism would need free oxygen as well (I think) so could not develop in the environment of the Early Earth.
Carbohydrates have the advantage of being nonvolatile, so can be stored and digested more easily than a trace gas or volatile liquid.
Long chain hydrocarbons are quite difficult to synthesis apparently - it takes a long time to make gasoline under extreme conditions- so there wouldn't be many sources of petrol around to feed our hypothetical fuel bugs.
Which incidentally aren't so very hypothetical after all; a number of different organisms are able to use diesel fuel for energy, as you may know; see here
http://www.fueldoctors.com/fuel.htm
but they rely on humans to collect the stuff for them.