Local community is a culture that you develop for centuries. If you don't know the Joe from two houses away, you don't know the Joe from two houses away, period.
I lived the last moments of the 'mahalle' culture in Istanbul, Turkey, held its hand while its cerebral activites stopped. Until a couple decades ago streets were full of kids and we didn't have any problem with it. Now we hardly let them out, we fear that they'll be kidnapped, fruited, etc., sometimes also realistically. How we came to this point?
The 'mahalle', that is the neighbourhood, is a culture developed over time and is based on the trust you derive from generations of sharing the same cultural and social sphere. This was disrupted three times in the last century, first by hostility towards the non-Turkish induced by nationalism, and its results, second, by mass migration towards the City caused by industrialisation, and lastly and currently with the 'walled communities' (1) and occidentialisation of work life, i.e. rapidly changing occupations and moving houses often. I guess the case for most of the US is similar (i.e. people don't get to share the same neighbourhood for that long), and under these circumstances a healthy local community is impossible to form, even if all the start-up eyes were turned on the problem, because it has a hard dependency on multigenerational relationships, and time spent together.
(1) a very interesting documentary by AlJazeera Turkey on this phenomenon exists.