They do age on the shelf, though, but generally not as fast as when in use. What happens is you have a rising probability of ICs being dead when unpackaged, or dying earlier when used. Several aspects degrade the ICs over time.
- radiation (both natural and man-made)
- chemical degradation of the dielectric
The effect of these influences largely depends on the manufacturing process and the quality of the IC. A well-made IC may be less prone to oxidation, for instance. Older ICs (aka larger structures) have more material to be eaten away. Some ICs have dielectrics than can be more prone to aging. Modern ICs seem to be built with thinner, but more robust dielectrics.