How is its edibility relevant? Being edible means it's in a certain configuration that the human "engine" can process. I don't think it's obvious that this configuration is inherently inefficient for non-biological engines, even if it is the case.
Well, there is the definition of modern argiculture, which goes something like:
"Modern agriculture is the process of converting hydrocarbons to carbohydrates."
In other words, the way we feed the large human population, is by leveraging a diminishing supply of fossil fuels to grow food. Replacing fossil fuels with food in that equation doesn't really work; we need to think of something else.
Not because making biofuel is impossible, but because one of the main uses for fuel would be to make food -- and making food and making biofuel is in direct competition for resources.
