Apple don't do sales --- sometimes resellers offer discounts, but direct from Apple, it doesnt happen. As for the reason, I'd say a couple;
a. They are a luxury brand (relative) and don't want to dilute their image by selling to the rabble so to speak. You'll also find this with designer clothes, that excess items are often thrown away rather than sold cheaply.
b. They don't have to. Their items sell at full price as it is, so why offer a discount? Leftover items after a refresh can be retained to be distributed as and when is needed for years to come as replacements.
c. Sales are counter-intuitive for many businesses. Apple has a constant flow of income, whereas a sale culture can disrupt that and cause buyers to hold off making purchases until sale time. If sales are never offered, customers cannot anticipate. (I personally also see this as one of the numerous reasons Apple likes to keep their refreshes hush hush).
Apple do sell refurbished items on their online store at a discount, and also some clearance items.
