well think about it; they live in a world where they are constantly hunted by grimm, and that one mineral source has let them fight them off for centuries. It only makes sense that they tried to make it into fuel. Plus it doesn't seem like it's in any real shortage, they even use it to make certain kinds of clothes apparently. So it's no wonder they use it for everything.



Plus; it IS a fictional world. it has it's own rules all together. who's to say they even know about how fossil fuels work?