If a dye used to print a textile is chemically locked in2 the fabric, why does the design fade over time?

I am not a fabric expert, but would imagine if kept dry, out of sunlight, and never laundered, they would not fade. Light, heat, detergents, and oxygen, themsleves, can fade pigments (color molecules), natural or artificial.. It does so with haircolor, fabric, paint, etcetera; I guess that is just the nature of pigments, it's probably an oxidation process.