there is a simple metallurgical answer.
Endurance limit.
Steel has one, aluminum and carbon composite doesn't. Without mechanical or weathering damage to alter it, steel will last forever.
The endurance limit is a stress value below which no cumulative damage is formed in the material. The material will not crack.
Aluminum will eventually crack with enough cycles (millions, etc.) at any stress. Same with carbon composite, and there are many ways to produce undetectable matrix damage in composite.