I am pretty sure that in the past ten years the proportion of the US population with a passport has risen at a faster rate than the proportion of the population with a passport in most OECD countries has risen.

That's what happens when: a) the proportion of the population in the US with a passport was starting off from a low percentage relative to other OECD countries' populations; and b) the US federal government changes the border crossing requirements and pushes other countries to act in more paranoid ways too.

When not even 6 out of 10 US persons take all their paid vacation days despite being given so few vacation days compared to most OECD countries, traveling to other countries (which requires a passport) just doesn't take place with the same frequency, especially when the US has a lot of domestic tourism options for all seasons to escape from all seasons which may be cheaper and perhaps even faster to reach.

Isn't it the case that the plurality of those c. 60% of Americans who take their full paid vacation are primarily those with children in their household? Staying on the road may be more economical than flying or taking a boat overseas.