Havana, Cuba was once a thriving, cosmopolitan city with extraordinary architecture and abundant natural resources. Detroit once had the highest per capita income of any city in the world. As their politics moved leftward, their cities turned into relics of civilization. Again, please see Andrew Moore's large format photographs in both of these cities for visual proof. They are in ruins.
Capitalists, on the other hand, make things nice and shiny and clean. It's simple: if someone owns something, they tend to take care of it. Those falling down buildings are generally "owned" by the city of New York, as their previous true owners were rent controlled into financial oblivion, and thus walked away from the properties.
What if all of those falling down buildings were fixed up so that all those poor people had a clean, safe place to live? If the government spent tax money on it, everyone would applaud; if a group of investors did it, all the whiners would complain about "greedy landlords."
So, the people stay poor, and the buildings keep crumbling. Way to go!