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Macs hold their value because other people want them and there are significantly more PCs out there. So, if more people buy Macs, then the demand is reduced for used Macs. Prices equilibrate. This is how capitalism works.
PCs are a better value because they are infinitely variable. Macs have a small set of components that are more expensive. The idea that you need to justify that Macs are cheaper suggests an inferiority complex on your part.