> I'm not sure in which way a US "passport card" differs from a plastic identity card. But it's notable that a few countries don't have either (UK doesn't... Russia and a few neighbouring countries, as mentioned before, instead have an "internal passport")

US simply doesn't have any official identity card.

So they make up replacements for when it's convenient to actually have one.

I suppose the goal is to encourage innovation in the identity theft industry... after all it adds to the PIB doesn't it?