Suburbia.

In the post-World War II baby boom and economic upswing, the US began a process of moving from cities to suburbs. Levittown was founded in 1947 outside of New York City and became the model for this kind of living. People flocked to it, particularly the affluent white people who drive American culture. (Levittown itself was restricted to Caucasians) I think that many cultural differences that non-Americans are wowed by come from this period.

The insulated individualism of Americans comes from selling cheap houses to individual families rather than large-scale apartment buildings.

This necessitated a car culture which fed back on itself. The suburbs took over farmland, so food growing needed to be centralized in the midwestern states. That meant long-haul trucking which meant more paved highways and ultimately the interstate highway system (which was also a civil defense structure for when the Russians invaded). This is why so many Americans can't drive standard-transmission cars. Because the model here is that the majority of a commute is on the highway.

Suburbia fed our sexual repression as well. Young people were restricted to where they could walk or bicycle until they could afford a car of their own. The closed-off nature of housing meant that any sexuality happened behind closed doors.

Even the red plastic cup that always comes up on Reddit threads as something that fascinates tourists in the US owes itself to suburbia. When an American family wants to get together with other families, rather than go out to a public park, they'll invite everyone to one house. With so many people, it doesn't make sense to use the best dishes, cups, and flatware, so disposables are much more popular.