Easy answer: its too hot.
The deepest man made hole is Kola Superdeep Borehole. That was a Russian experiment to see how deep you could drill into earths crust, reaching down 12,262 meters. At that depth the temperature reached 180 °C. The scientists estimated that a 15,000 meters depth the temperature would reach 300 °C, and at that temperature the drill would cease to work and the project was stopped.
