There are two reasons. First, when the earth formed 4500 million years ago, it was molten throughout...a giant magma ocean!. It has been cooling ever since by losing heat through the surface. The second reason why its still hot is that certain radioactive elements like uranium and potassium spontaneously decay to other elements (lead and argon) in radioactive decay. When the parent element decays to the daughter element, HEAT is generated.

The interior of earth is hot because of a process called radioactive decay. Certain elements found in Earth are unstable. Through time, an atom of these elements will "kick out" or "lose" protons or neutrons. (Remember that atoms are made up of protons, neutrons, and electrons.) This change produces energy in the form of heat. Uranium is one element that undergoes radioactive decay to produce heat in the Earth.

The interior of the Earth is hot for several reasons and much of the heat is left over from the formation of the earth 4.6 billion years ago. The earth formed out of a spinning mass of gases and meteorites that were drawn together by the force of gravity. As they collided together into the earth,they produced a tremendous amount of heat.

The earth has cooled quite a bit, but it is still as much as 4300C (7770F) in the middle. The material the earth is made of also contains many radioactive elements. As these elements decay, they release more energy. So the interior of the earth is hot because of gravitational energy, impact energy, and radioactive energy.