Here's why:

* Radioactive Decay: Uranium is a radioactive element. This means its atoms are unstable and spontaneously decay over time, releasing energy in the process.

* Heat Generation: This energy release takes the form of heat.

* Earth's Internal Heat: The heat generated by radioactive decay in the Earth's mantle and crust contributes to the Earth's internal heat. This heat drives processes like plate tectonics, volcanism, and the Earth's magnetic field.

While other sources contribute to Earth's internal heat (like leftover heat from Earth's formation), radioactive decay of elements like uranium is a major ongoing source.