One of many reasons why per capita CO₂ emissions in US are 14.9t while in Germany, country that is always mentioned because they closed nuclear plants it is 'only' 8t (which is still higher than EU average).
Families tend to get spread out and vacations tend to be very short. There is a strong encouragement to meet for recognized holidays, so these are by far the busiest times at airports.
Last year, the prediction was 4.7 million people in the US traveling by plane over the thanksgiving holiday, which demolishes the 1% comment immediately.
You are greatly underestimating how common it is for Americans to travel by plane. Almost half of Americans fly at least once a year. It is sufficiently inexpensive that almost everyone can readily afford to.
A little bit of both, and they are related. Trains are a tough sell because they aren't competitive for most travel. Even at 300kph, they're only good for local-ish travel (by that, I mean up to perhaps as much as 1000km). Would be great for Portland to Seattle, or Portland to LA, but if you're going out of region (which is extremely common), an airplane will be way faster and almost certainly cheaper too.
I'd love a moderately fast train, say 200kph, between cities like Portland and Seattle. That's a great use case.
But as a nationwide network, there won't ever be a suitable rail alternative, unless it gets subsidized. Amtrak is already stupidly expensive for what you get.
Except, they do in a country like the US that has massive distances between cities.