I am not sure how true this is, but even if it were, I would assume you're largely referring to European countries, where the nature of air travel is very diffe

I am not sure how true this is, but even if it were, I would assume you're largely referring to European countries, where the nature of air travel is very different. In most of Europe you cannot get on a plane and fly in any direction for more than an hour or two and not wind up in a different country. Hence people who fly are overwhelmingly going to have a passport if it's needed (especially if you factor in that many European countries have excellent high speed rail for intra-country travel), and therefore this barrier is not much of an issue.