To a large degree we're still living in the shadow of the industrial revolution, when workers flocked to cities for work. As a telecommuter, my quality of life is a thousand fold higher than my co-workers in the D.C. or Dallas metro areas where it seems many people find themselves alone in a sea of humans and chronically maligning poorly planned mass transit.
Sources for greater economic growth in urban areas:
https://www.ers.usda.gov/topics/rural-economy-population/emp...
Countries by CO2 emissions per-capita: https://en.wikipedia.org/wiki/List_of_countries_by_carbon_di...
Countries by urbanization: https://en.wikipedia.org/wiki/Urbanization_by_country
And that's not even getting into things like the amount of land that gets developed for low density homes and services, roads, water reservoirs, electric lines, etc.
Wealthy rural countries (USA, Australia, Russia), produce more carbon than dense wealthy countries. Granted, I wouldn't be surprised if carbon emissions and environmental impact of the average city resident was higher than the average rural resident - but that's largely a result of the fact that rural residents are generally less wealthy. This is definitely true on a global scale (the countries with the lowest carbon footprints are poor, rural ones), and I wouldn't be surprised if it were true on a nation wide scale. I didn't explicitly state it in in the first post, but I did clarify in the second that this was for the same standard of living.
On a side note, an interesting thing to look into is how Singapore manages to provide so much services with such a small budget - 14.2% of GDP per capita as opposed to 26% in the US. A huge part of this is because it's so much more efficient to provide services when you're dealing with population concentrated in such a small area. Imagine how much more money (and energy) it takes to deliver electricity, running water, sewage, emergency services, education, etc. to households in Oregon where 4 million people are distributed across 98,466 square miles as opposed to concentrating them all in 278 square miles. Even if things like land and labor cost more, so much overhead and operational costs are saved by not having to service such a massive area.
I realize that's more of a value judgement and not objective. But there are definitely costs associated with living in dense urban places. Regardless of efficiencies we are definitely still living in a post-industrial-revolution world, and people did not voluntarily cram themselves into giant cities then, so some sort of reversion when it's possible makes sense. Vast populations of urban people in China would rather not have to move into the city.
This isn't the case. Silicon Valley, Seattle, etc. are just on the leading edge of a larger tend towards greater urbanization. It's a pattern that's also happening in Austin, Portland, and more cities just to less extreme degrees. The simple fact is that in a developed country dense population centers are more efficient - both economically and environmentally.