The thing is that a lot of suburban areas are much younger than high-density areas. So they have borrowed for capital projects such as infrastructure and new buildings. They aren't paying the true costs of maintaining those things yet.

Cities on the other hand have to pay for the upkeep on their existing infrastructure, as well as replacing that infrastructure.

Replacing infrastructure is much more expensive than building it out in the first place. A lot of suburbs will fail when it comes time to replace their infrastructure. It just hasn't happened yet because in the United States you can keep building out forever. Furthermore people move away from areas burdened by the costs of replacing infrastructure.

The entire rust belt is being brutalized by the fact that formerly suburban areas have collapsed. The urban areas were already brutalized in the 50s, 60s and 70s by white flight.