I think it's more that communities were smaller and more consistent. People didn't move that much. Your neighbors were your lifelong neighbors, and the postman was your lifelong postman, so you knew him, and your neighbors knew him, so they didn't lack trust in anybody—they had trust in their local "social network," both toward specific individuals, and because of the implicit "peer pressure" that kept everybody in line. I.e the postman would have not been able to run away with your child without the whole community being after him.