It's absolutely the built environment. Your Costco example is a clear example. you drive to Costco, you walk in, grab generic packaged goods without needing to really talk to anyone, and then go to the checkout and use the automated kiosk to make your purchase.

There's no reason to have a human interaction, so why would you bother getting to know the cashier? You're never going to build a relationship with the cashier precisely because of the environmental structure.

Contrast that with walking down the street to a local store that one of your neighbors owns. I bet you would already have a relationship unless you chose not to. Why? Because you'd also see them at your kids birthday party, or you'd see them at the bark down the street, or out on a walk.