Because nobody wants to live like Appalachians when they can live like San Franciscans. One is clearly more comfortable.

Don't city dwellers in general consume less greenhouse gases due to improved density and efficiency?

The improved density and efficienty allows population that necessitates the use of factory farming, petrochemical based fertilizers, massive infrastructure, traffic congestion etc. Per individual the carbon footprint is smaller (until you get to the really rural poor), but overall the effect of this process is more consumption on net. It's just the way it is. If we really wanted to go green, we'd all be on mopeds and have one shirt.

You're making a Malthusian argument. Yes, if you were to kill the city dwellers, then net carbon release would be lower. Given the same amount of humans, however, distributed with rural or suburban densities, you face a dramatic increase in carbon release. Unless humans were to go back to pre-industrial populations, carbon release would be much higher.

I think that rural life is more enjoyable than urban life, but feelings don't change the facts.

So I'm not sure what you're advocating. It is almost as if you are changing the argument in order to feel as if you won.

I'm not advocating anything.

And we'd also have no kids, because that's an exponential factor when it comes to pollution.

Kids are only an exponential factor if you have a constant number per adult each generation.

Or if people live longer and longer, which some very smart folks are actively working on.