Why are coal, oil, and natural gas called fossil fuels?

Because that's where they come from. Coal, oil, and natural gas are the products

of dead dinosaurs rotting in the ground under great pressure for millions of years.

Coal, oil and natural gas are are called fossil fuels because they are composed of the remains of organisms that lived as long as 500 million years ago. These organisms, such as phytoplankton, became incorporated into the bottom sediments and then were converted, with time, to oil and gas.