Thank you for the opportunity to help you with your question!
They are called " fossil fuels" because like fossils of shells and plants which are found in rocks ,they are old often hundreds of millions of years ago. Fossil fuels are part of the remains of living things which
once flourished on the planet, but died and became buried under thick layers
of younger rocks.
Please let me know if you need any clarification. I'm always happy to answer your questions.Give me a good reputation please.