Compounds that are used to manufacture tires' shells, bonds between layers, and the structural elements can deteriorate over time.

Also tires will tend to become less flexible over time from lack of use. Using tires flexes and applies forces that prevent tires from sagging. Also, flat spots -- although somewhat eliminated with today's compounds --  can develop from laying down on their sides or being stacked and not being repositioned.

Tire technology is advancing. Costs for new tires are not likely to increase greatly over the next years relative to the advancement in performance and durability. It is likely that buying tires today will lock you in to using an old technology and tires that are deteriorating.

Being locked into stored tire will make it impossible for you to take advantage of any new technology that could come available in the next few years.