I would argue differently, and say that it's entirely a factor of density. The more people that are crammed into a small area, the more you will see the spectrum of human behavior in any given situation.
So, in the midwest you might see people clearing sidewalks and driveways for each other - I'm more familiar with the HOA that sends threatening letters to people for not shoveling fast enough. But you don't see both those things at the same time, so you can choose to focus on whichever you prefer.
In my experience in a large US city, people are incredibly kind and very much look out for each other, even strangers. I have many times seen strangers helping each other in situations, both minor and serious, and neighbors even more so. And sure, yes, I've seen people acting badly, but in general from living in both part of the country, people in the city are far and away nicer and more concerned about the welfare of those around them.