The problem is that the huge suburban "exclusion zone" around cities means that central parts of the metro are very far from open green space. Compare e.g. the development pattern around Cologne-Essen:
https://www.google.com/maps/place/Cologne,+Germany/@51.20163...
to Houston:
https://www.google.com/maps/place/Houston,+TX/@29.759369,-95...
In the one case you are never more than five miles from green space, in the other you may be 20+ in the middle. It's a tragedy of the commons -- no one driveway thinks it caused the flood (literally, in some cases!). Even Paris is more compact than Houston despite having 2x the population!
This can affect air quality, urban heat islands, mental health, etc.