Prestige. And finally I come to the most important factor when it comes to name brands. This is what brands spend the major money on; associating the brand with prestige, with a good life, with a luxury lifestyle. Why buy the off-brand sugar water when you can buy Coca Cola and its Polar Bears? Don't you know that buying off-brand sugar water kills Santa? But you usually see this effect more with true luxury goods; if you've ever been in the market for a luxury car, expensive bottle of booze, high-end leather goods, watches, or anything, you understand that you are paying partly for the increased cost of production, but partly for the exclusivity; only certain people can afford to have it. Prestige matters. Buying an off-brand Rolex is cheaper than a real Rolex, and drinking good sippin' tequila will cost you a lot more than drinking Mr. Boston's well liquor.
D&D emerged in the 1970s as a hobbyist game. TSR was late to the idea of selling adventures because, in their estimation, what D&D gamer would want to run someone else's adventures? It used to be common to mix & match between official product, different rulesets (OD&D, B/X, AD&D, Boot Hill, Gamma World), semi-official product (Dragon Magazine), homebrew, 3PP (The Compleat Alchemist, Arduin Grimoire, Grimtooth's, etc.) and so on.