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.
Meanwhile, in this universe, travel inside the Schengen area (which covers an impressive chunk of Europe) does not require a passport, but any kind of national ID, regardless how many borders it crosses. Coincidentally, it's the same as for internal air travel.
I guessed as much, thanks.
Domestic flights within the UK don't require passports (although the airlines may require some form of ID to be presented in order to get boarding passes).
No, many European countries have national ID cards and you can fly between some EU countries using only that, not your passport.