Why is the core of the earth made in metal?

Answer 1

Metal sinks to the center because it is denser than magma.

We don't really know the "why" of anything. We describe things including processes.

Answer 2

The core of the Earth is predominantly made of iron and nickel because these elements were present in the materials that accreted to form the early Earth. During the Earth's formation, heavy elements like iron sank towards the center due to gravitational forces, while lighter elements rose towards the surface. This process resulted in the concentration of iron and nickel in the core.