Pick one item for sale in Walmart (Target if you are more enlightened than others). Now tell me how you would get that item from the point of manufacture to the shelves of every store in the country. Detail the transportation methods used. Also make note of the federal, state and local laws that have to be followed. Arrange for the hiring of everyone involved, and the acquiring of the infrastructure and vehicles needed to facilitate the transport. Enumerate the methods you would use to ensure that a steady supply of the item is obtained, without having to store excess inventory.
Now tell me why you think it's just as hard to open a box and put that item on a shelf.