Pet Peeve #3472

How come whenever I go to a store to buy a common item, 50% of the time, they are out of it. It can be something as simple as an aspirin at a drug store, or a type of candy at a colmado. I don't get it. When I buy something and I realize that it is the last or almost the last one in stock I always make a point of telling the person working there that they are almost out of the product. Doesn't do any good. The next time I go there to buy it, they not only don't they have the product but they act as if they are not supposed to know that, even though I told them.