Zoning. By itself, the suburbs are fine. But it's the zoning that goes out of its way to pave all that greenery you theoretically move out to see, that causes

Zoning. By itself, the suburbs are fine. But it's the zoning that goes out of its way to pave all that greenery you theoretically move out to see, that causes the problems. No mixed-use zoning clusters the lower-income together into apartment complexes and neighborhoods that have a net negative social effect. Parking lot requirements spread everything out to nigh-unwalkable distances. Varying or non-existent sidewalk requirements finish off any chance of walkability, or even bike-ability. All that space makes efficient mass transit implausible, forcing poorer/younger residents to lock up a significant chunk of their expenses in their cars, and making car trouble a near-catastrophic point of failure. All that space and all those cars also create traffic-related waste that's largely impossible to avoid and very expensive to scale with growth. Cul-de-sac neighborhoods kill any chance at efficient traffic flow. The spacing leads to higher infrastructure costs per resident (higher taxes) which retards or suffocates competition and advancement in the infrastructure space. Etc. In short: suburbs are great place if you're relatively wealthy and they aren't too big yet. But they're not self-sufficient or healthy socio-economic communities.