Is it? It always seemed like more of a cash flow question to me. You buy whatever you are going to use, put it at the back of the shelf, and grab the thing you bought 3 months ago. Clearly, we're talking about only non-perishables here. But the goal of a 3 month supply is not a hoard in a closet that is pulled out in an emergency, but a flow of goods through your normal life. You build up to the supply a bit at a time, so it is never a full 3 month purchase, but a slow steady growth until you hit your goal.
To add to this: Buy in bulk when it goes on sale and replace individual units as consumed. No need to buy it all right away when you can slowly acquire bulk bargains whenever the opportunity arises.