No disrespect taken!
I think you may be intuitively underestimating the size of the US. I live in Arkansas, which is a fairly mid-sized state. Driving across it at highway speed (70mph/115kph) E/W or N/S takes about four hours. It’s slightly larger than England in area.
Consider someone who lives in Austin, TX. They could drive four hours in any direction and still be in Texas. The US as a whole is approximately the size of all of Europe.
Given that kind of scale, it seems illogical to say that many Americans have “never experienced a temperature outside of their own” merely because we tend to not travel internationally as often as others.
As for comfort zones - I think that’s something of a funny thing. I’ve been to Montreal a couple of times, and honestly felt more at ease there than when visiting New York City, Los Angeles, or Hawaii. There is a huge amount of diversity in terms of culture within the United States.
The other big factor is expense. My wife and I plan to visit Scotland and Italy at some point. We estimated that a week in Scotland for the two of us would cost about $7,000 - $5,000 of which is airfare alone! Consider that the median household income in the US is about $60,000 ($50,000 after federal income taxes). That week in Scotland is approximately the equivalent of two months’ income for a median household! In comparison, a round-trip flight from Heathrow to Istanbul is $330. It’s much, much cheaper for Europeans to travel internationally.
