Flying anywhere is cost prohibitive for a lot of families in the US. Buying 4 or 5 airline tickets gets expensive fast, even on low-cost airlines. The median family vacation in the US is probably a car trip to a destination within 4-5 hours of where the family lives.
This isn't as relevant for people without families, but many of them tend to be early in their career, so money and vacation time are bigger constraints.
Median family income in households with children in the US is ~$80,000/year, mean is ~$100,500/year[1]. That's enough to afford $1,000 in airline tickets if you wanted to. Millions of people fly to kid-centric destinations like Orlando, Florida a year, and Orlando is incredibly cheap to fly to. It also amazes me how many people bring children on cruises and they even have kid centric cruises. The cost to fly is also at a historic low.[2][3] I see routes I took 10 years ago not increase in price since then.
Most people I know are not "travel people" and wouldn't take a "travel vacation" if it was free, so they are not buying airline tickets. Just cuz they aren't buying airline tickets doesn't mean that can't afford them if they wanted to.
My point was, if you're going to get onto an airplane it doesn't have to cost you significantly more to travel to a foreign country. Also traveling abroad isn't exactly something one "should" do, some people just don't want to.
[1] https://www.census.gov/data/tables/time-series/demo/income-p...
[2] https://www.cntraveler.com/story/average-domestic-airfares-n...
[3] https://sfgate.com/chris-mcginnis/article/Hawaii-South-Afric...
It leads to an uneducated populace, ignorant of other cultures and with a very limited perspective of world affairs.
People's understanding of the world becomes limited to what you hear from the state department and the news media.
Makes it easy to drop bombs on people a world away if you never bothered to get to know the people or places you're destroying.
You can learn about foreign affairs and become educated without visiting other counties, it's not a requirement. You'll learn more in books than you'll learn drinking a margarita in Cancun resort.
Visiting other countries doesn't, by definition, lead to learning about foreign affairs and becoming educated of other cultures. I studied abroad and some of my fellow study abroad students didn't develop any understanding of foreign affairs or enlightenment about culture.
No place that has been bombed lately are where tourists will travel to.
It's not cost prohibited, discount airlines like Norwegian Air and Air Lingus run non-stop flights from the East coast to Europe for $100-$300. Flights to the Caribbean are not more expensive than domestic flights.