The markup is because Apple can command it based on a long history of quality and building a tech luxury brand. It's solely about keeping margins high and the brand status high.
Even if they could make the same money by lowering prices (and increasing volume), it'd be a terrible idea based on how consumer behavior and status seeking actually works. If the quality were the same and the product were far cheaper, consumers wouldn't want it as much. The absolute worst place to be in any market tends to be the middle, you go to the middle to die. High margins provide a margin for error in business, it's invaluable.
Even if they could make the same money by lowering prices (and increasing volume), it'd be a terrible idea based on how consumer behavior and status seeking actually works. If the quality were the same and the product were far cheaper, consumers wouldn't want it as much. The absolute worst place to be in any market tends to be the middle, you go to the middle to die. High margins provide a margin for error in business, it's invaluable.