WHY doesn't the Earth's core freeze?

 The core is the source of the magnetic field that created life. 

It's actually cooling down, just slowly. From the Earth's core, colossal upward thermal flows of magma, the plumes, rise. At the surface of the mantle, they spread out to the sides, causing continents to drift, and when they get cold, they descend into the depths. 

Yet the core also contains heating sources: the decay of long-lived radioactive elements and the friction between the core and the Earth's outer layers, which tides gradually slow down the rotation of the Earth. 

As per estimated computations, something like 1 billion years would pass before the core chills off totally and chemical and different responses in it stop.