[jiggawatts]

Metallic asteroids are mostly nickel-iron.

On Earth, geological processes concentrate elements into ores, primarily through volcanic and hydrological means. Neither are available in small, cold asteroids devoid of liquid water. Hence asteroids are generally undifferentiated mineralogically, making mining them much less economically viable.

You often see total quantities listed as an amazing thing, glossing over the fact that the Earth has more of everything and in usefully concentrated lumps.