I feel like I run into microcosms of this in a few online games.
Worlds like Sea of Thieves, The Division's dark zone, and Stalcraft, are built with the idea that "anything goes" - players exist in the same world, with no rule to prevent them killing each other to steal their possessions - and even some decent rewards for doing so.
I actually mostly enjoy playing those games for all the times people don't do those things. I don't despair the moments that betrayal does end up happening - mostly, I just find it wonderous and satisfying anytime we manage to dismiss that possibility and treat each other peacefully.
This could be a poor effort to correlate my interests, but one thing I think affects this issue in places like America is cars. You don't see 20 people out on the street. You see 20 cars on the street. Tinted windows, faceless metal grill. A lot of people have been burned by one poor experience with neighbors taking sidewalks or transit, and so they want to stay isolated in their own protected cabins.
I think the world really relies on chance interactions between strangers, for both parties to learn something about each other and the world - often leading people to "care more" and develop more of that social contract. The trick is, most people DO follow a social contract, but it might only be for the individuals they're familiar with and that they feel similar to. The internet has unfortunately had its ill effects too - people can choose to stay in spheres where people specifically agree with their worldview, and won't ever run into "randomized neighbors" the same way as they would walking down the street.