I think the problem is that the US essentially spans from the arctic circle down close to the tropic of cancer - if you include Puerto Rico it extends past.
This provides a large number of environments within the US. It's hard to blame someone for not going through the colossal bullshit of an international crossing when you can simply drive or take a national flight to get to another environment.
In countries like the UK (where I'm from) people travel out of country on almost every holiday as you want to go somewhere different, but it's hard to do in the UK.
I think there's a stronger correlation between vertical size of the country and lack of international travel.
Probably a more sensible comparison is to compare Europeans who travel out of Europe and Americans who travel out of the United States - both are roughly the same area.
[NB I mean Europe the Continent, not the EU, or Western Europe]
I believe that is a fairer comparison. My family traveled less distance to holiday in France than it takes my in-laws to visit much of their family whilst remaining within Canada and that's only Ontario to the Atlantic's, that's nothing compared to some of their family that lives on the Pacific and can travel to the Atlantic just to see family.
However I have currently traveled to the US and Turkey (in the legitimately Asian geographical area), and currently live in Canada and am currently planning a trip from Canada to Australia because of my brother. Which will have netted me 4 continents. I'll need to hit up somewhere in South America and Africa, perhaps I'll have to land myself in Japan or China sometime to fully claim Asia.
Simple economics is most of the reason Americans dont travel internationally very often. You can take a RyanAir flight from the UK almost anywhere in Europe for a fraction of the cost of any international flight out of the US. Americans who don't live on the coast can pay more to get to a hub airport than Europeans will spend getting to their destination.
This provides a large number of environments within the US. It's hard to blame someone for not going through the colossal bullshit of an international crossing when you can simply drive or take a national flight to get to another environment.
In countries like the UK (where I'm from) people travel out of country on almost every holiday as you want to go somewhere different, but it's hard to do in the UK.
I think there's a stronger correlation between vertical size of the country and lack of international travel.