I think it's the same way people get angry over extreme hobbies like free solo climbing. You're watching people squander something you place high value on. In free solo it's risking their lives for no easily understood reason.

In fashion, when someone drops 2k on a deeply ugly/plain outfit or shoe (that they only wear once to add insult to injury) and that money could have paid most people's rent or mortgage for the month or they could have fed and housed a hungry child, then I think the blatant, flaunting of economic inequality becomes disgusting and infuriating for many.

The same effect can be achieved by going to skid row with a million dollars and then balling up the bills one by one and throwing it into a bonfire in front of the poor.