From what I understand—I don't know this personally, but this is what I hear from the business people—they don't offer identical services when it comes to the shipping and receiving of goods. I may be misled on that, but this is my understanding.
Another thing they don't like is to have to be in a retail outlet where they often have to stand in line. They're busy. They're just trying to run their business, and for them, in their mind, the quickest and most efficient way is to go to the post office.