First of all Amazon isn't the only place to buy the product. Second,there are tons of companies that sell the same items, be it diapers or computers, on Amazon and they are obviously making good money on it or they wouldn't be doing it.
The difference it that a computer is something you might buy every few years but diapers are something you need day, after day, after day and you need to keep buying more. Ask a parent with a kid or 2 in diapers if they are spending more on diapers or computers.