Because you have needs, and some of these needs can not wait and needs to be satisfied now. Some of your needs may be ignored for a while though, and you'll be reluctant to spend on satisfying them. Yet, you'll decide to spend eventually (if you ever really wanted to buy whatever satisfies the postponed needs). That's because we have only that much time in our life (I doubt hoarding in itself is that rewarding on our personal level).

Also, the deflationary money may not be the only thing that accumulates value. A piece of land in a developing place may start accumulating value more rapidly than the money does. Won't you decide to buy if the item bought looks like a better investment compared to just holding money?