It's called choice and Free Market. It's not a hard concept to understand, but often you can buy something cheaper from the manufacturer, sometimes it's cheaper from a third party.
Apple is a monopoly on their platform, something MS got sued for and lost in the 90's because they bundled Internet Explorer with their operating system. They didn't force consumers to use it and there were other choices.
Apple does not allow the opportunity for choice and it is all artificially limited, making them a monopoly.
Actually most times it’s more expensive to buy from the manufacturer as opposed to a third party reseller. The manufacturer doesn’t want to undercut their dealers and resellers so typically the manufacturer sells for MSRP all while 3rd parties sell for cheaper.