Then mass suburbanization happened in the 60s/70s. American cities became high crime places. Everyone became anonymous. No one knew their neighbor.
Such is the paradox of modern urban life. Nowhere are you closer physically to your neighbor, but more distant socially. The 5 acre farms outside of town all know each other’s grandkids by name. Does a city dweller even know the name of the resident across the hall?
Here in Tokyo, it's still like this: kids play outside and roam around, taking the subway by themselves, etc. But people don't know their neighbors here either, since it's a city with tens of millions of people; it's just safe because it's built into the culture, just like petty theft almost never happens here and when you drop your wallet, it's almost certain to be turned into the nearest police box, with all the cash still inside.
There are many neighborhoods in cities and towns in the US where this is still true. I know because I live in one, and I've visited others. There are also a ton of US suburbs and exurbs where people barely know their neighbours.
Having a friendly neighborhood has much to do with the strength of community institutions and the existence of "third places". Those can be present (or not) in a variety of community layouts and densities.
My take is that it requires a sweet spot of economic security where people aren't struggling so much that they can't/don't trust in community, but also aren't so wealthy that they don't need/rely on their community. Beyond that, it also helps to have physical layouts that enable friendly unintentional encounters between residents.
The problem is that none of these places will be cheap to live in, because all else equal, the existence of that lifestyle will drive up demand (and therefore housing prices).
I didn't say that urban areas are less neighbourly.
I meant that the range of densities that are conducive to neighbourliness is a necessary, but insufficient condition.
People on average need to feel both economically secure enough and at the same time so wealthy as to not rely on anyone else.
They also need a third-place in which to congregate
This is why American suburbs outside the north east are so soul crushing - the "third place" is typically a church instead of a a pub.
Other places that come to mind include Ann Arbor MI, Burlington VT, Boulder CO.
Like I said, none of these are cheap places to live relative to nearby areas.