Do we though? When was the last time you rang up a supermarket and demanded cheaper products?
Competition between companies vying for our business drives down prices, but we don't 'demand' it - we just pay whatever price we are being charged on the shelf.
The problem with the Supermarket / Farmer relationship is just that Supermarkets have got to powerful and Farmers have little option but to sell their goods to them, giving the consumer (the Supermarket) the leverage in that situation to dictate the price rather than the Supplier.
When you see a BOGOF offer, it's run by the Supermarket but they make the Supplier pay for it, who has no choice.