Because we don't and we shouldn't? Don't defend bad practice in discussing the US by inventing bad practice that people you just made up are using to discuss Canada, Russia, and China.

> provides an excuse for facing the reality that America is not actually a very good place to live unless you are very wealthy?

You are literally insisting here that aggregate data conceals differences between groups of people. The end of your sentence angrily argues against the beginning of your sentence.

edit: the reason we need to disaggregate is because we need to talk about Mississippi. We need to talk about black America. We need to talk about Chicagoland separately from downstate Illinois. We need to talk about black Chicago separately from white Chicago. Aggregation helps us avoid things.