The real reason is that they have limited supply, and people want them. (people may not want bitcoin in the future btw) People always want gold and they want to gather as much as possible in their lifetime. It is the only solid asset which holds its true value through the centuries.
What about when we start asteroid mining or surveying asteroids with the future threat of mining? there are asteroids in our solar system that contain more metals/gold than we have mined here on earth..