The Earth's core is not a gas primarily due to the immense pressure exerted by the overlying layers of the Earth. This pressure raises the boiling points of metals, preventing them from transitioning into a gaseous state despite the high temperatures. The core is composed mainly of iron and nickel, which remain solid and liquid under these conditions. Additionally, the unique combination of temperature and pressure creates a dense environment that stabilizes the core's metallic state. Thus, the physical properties of materials under extreme conditions dictate the core's composition.