Here's why:

* Fossil fuels are formed from the remains of ancient organisms (plants and animals) over millions of years. They are primarily composed of carbon, hydrogen, and oxygen.

* Uranium is a naturally occurring radioactive element found in the Earth's crust. It's not derived from organic matter and doesn't contain the same chemical composition as fossil fuels.

While both uranium and fossil fuels are energy sources, they have very different origins and mechanisms for energy production.