I mean yes, they kind of are? Building codes vary widely from state to state and while some newly constructed homes in the US do feature more insulation than older buildings, there’s still room for improvement. Coming from Europe, most US homes would not be considered insulated.

See this article from German engineering magazine www.ingenieur.de (Google translated):

“Core insulation is possible in both new and existing buildings, provided that the outer wall is or was constructed from two shells. In a new building, two shells means that there is a stronger load-bearing wall that also functions as a heat store. It is 18 to 25 centimeters thick. The second shell provides weather protection, is eight to twelve centimeters thick and is anchored to the inner wall. In between is the insulation layer, which is ideally 15 to 25 centimeters thick.”

And that’s without outside/inside plastering / paneling. So insulation plus outer walls is typically 50cm+. Here in Germany nowadays it tends to be even more than that because a) we still don’t believe in AC for whatever reason and so thick walls = at least some heat protection and b) home construction subsidies are usually tied to more demanding standards than the legal baseline.

So TLDR: yes, US homes do have some insulation wedged between its timber frames, but elsewhere building codes are much stricter