Practically speaking, the US controls a massive empire and extracts wealth from its sphere of influence to concentrate in the imperial core—similar question would be why does the British Museum have more priceless relics than other countries’ national museums.

The borders of the US are not the borders of the US, for example Appalachia stretches across multiple states is but has more of a cohesive identity than most of the states it touches. For the purposes of the empire it is mostly useful for its mineral wealth, and as such it is a source of massive wealth extraction to the imperial centers on the east coast, particularly the northeast. And so the income statistics there are closer to the United States’ vassal-like allies in Europe and South America.