You're right that the US is a fabulously wealthy country. Still, this might be a misguided comparison: the least developed region of NZ still has a higher HDI than two US states[1][2], without considering territories.

If you decided to wander around West Virginia (as I have), you'd see plenty of people burning coal and/or firewood for heat in winter (and WV winters are much harsher than NZ ones, from what I know about NZ). You'll also see extreme indigence of a sort that's otherwise almost completely invisible in the US.

[1]: https://en.wikipedia.org/wiki/List_of_U.S._states_and_territ...

[2]: https://en.wikipedia.org/wiki/List_of_regions_of_New_Zealand...