That's not a truism. If manufacturers only sell DTC (direct to consumer) and don't allow third party sellers to compete they do so for two reasons, price control and brand control. The lack of competition between third party sellers allows them to sell at any price the market will bare since they are the only ones you can buy the product from. When third party sellers can distribute/sell the product as well and there is no enforced MAP (minimum allowed price) it leads to a race to who is willing to accept the smallest margins on the sales, which in turn makes it purely a who can do the most volume with the best marketing. This actually results in the lowest prices for consumers.