Internal combustion engines and humans fundamentally use the same chemical process to generate energy. The fact that something can be used as automotive fuel alone says nothing about whether or not it is safe for human consumption.
Different process, same outcome: hydrocarbons are broken down and oxidized into CO2. We just do it with some enzymes in the Krebs cycle instead of doing a high temperature reaction.
Of glucose, not a hydrocarbon, but there are plenty of organisms that use hydrocarbons directly.
We don't because we use glucose as our easily transportable fuel, which we evolved because plants happened to produce glucose when we evolved. If there were plants producing some hydrocarbon in fruits we'd have evolved mitochondria to use that instead.