@turquoise, @simrin-- I suppose "dry goods" can be kind of confusing. It's probably better to call them "soft goods" instead. As far as I know, "soft" refers to the type of material the goods are made of, that is-- soft material. Cloth, leather, paper and plastic, for example, are all soft goods.
The reason that soft / dry goods are considered to have short shelf lives is because these are goods which are bought often. They are non-durable goods meaning that they wear out quickly. Durable goods, on the other hand, last longer and are not purchased as often.
For example, we buy dry goods like clothing fairly often, but we don't buy cars that often. Cars and appliances are durable goods for that reason, they have a longer "shelf-life."