Population isn’t evenly distributed. Iowa has a little over 1/3 or NYC’s population even excluding everything over the Hudson. Your “boom towns” represent a large chunk of everyone in the US.
Lots of rural areas and small cities have broadly shared prosperity too. I was very surprised when I went to East Texas and saw the brand new strip malls and shiny pickup trucks everywhere. (Along with a very racially diverse and integrated population driving those trucks and eating at the new big box restaurants. I was completely unsurprised to see Trump doing better than expected among Latinos in Texas.)
NYC, DC, and SF are probably the only cities I’d put in that category. Maybe Seattle. That’s only 12% of the population.
The median household income in San Francisco is $120,000, but the median home price is $1.5-2 million, depending on source. That's 12-16x.
A $500/month truck payment isn't putting people on the edge of financial ruin when you're talking about an $800/month mortgage versus a $6,500/month mortgage.