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Living organisms are very complex and the only reason the fossils of old animals and plants are used is because the pressure and heat of decomposition has broken the chains down into simpler compounds that are more easily utilized.
Cause when the materials aren't broken down they need to be fracked, and that's a pain in the ass because you basically need to heat the shit out of them to break them down and then it's less efficient because using energy to create energy is eh
Like all living things are just carbon chains that came from somewhere else