We found out, soon as communities grow above about 500 or so. It lined up with: https://en.wikipedia.org/wiki/Dunbar's_number
In effect, something in our psychology wants us to know everyone. When you live in a city with 100,000 people. You basically break mentally.
This is why places like Paris have been designed around living in your neighbourhood. Why there's more and more focus about the neighbourhood government being the sovereign most government.