Living in a big city is usually better for the environment than living in a rural area. City-dwellers live in smaller spaces closer together, so they consume fewer resources and emit less carbon.
[1] https://www.eia.gov/consumption/residential/data/2015/c&e/pd... (See site energy consumption per household member by housing type on page 2).
So many people, potentially desparate people, concentrated in one place, utterly dependent on supplies being shipped in from elsewhere.
And we can't have everyone living ever-smaller lives in ever-more-dense cities anyway, as you need all the food production, manufacturing, energy production, and resource extraction to keep those cities alive. And for now, that still requires a lot of human labour (far too much of it overseas, given increasing geopolitical instability)