Science!

[Alex]

It is oxidation (even when in use, the actual shaving is not a significant source of dulling as much as the water exposure). Dosn't take a lot to significantly dull a fine edge. And bathrooms, where most people keep their razors are high humidity environments.

This is why my newly opened back of blades (which I bought perhaps 18 months ago are still completely fine, they've been kept dry and away from humidity).

This is also considered an explanation for why those mystical pyramids you can buy appear to help keep razors sharp. The instructions tell you to dry the blade and then you put it under something which protects it from ambient humidity.