Probably because that requires a ton of logistics in order to build up and store the reserve, not to mention whatever you're imagining is done with the reserve in order to discourage price gouging.
I'm not sure what you think is supposed to be so difficult about the logistics. When regular supplies of certain good fall off the government would just sell from its stockpiles to increase the market supply and prevent gouging.
I still don't understand. Why wouldn't retailers buy from government stockpiles when their regular suppliers are short? The idea is to make up for the shortage so there's no basis for gouging.