[pluma]

It's not uniquely illegal (at least in Germany). It's illegal for the same reason forging any document (for fraudulent purposes -- i.e. in 99% of all cases) is illegal: it misrepresents an authoritative claim.

It's not entirely unlike having a friend show up in a fake police uniform to vouch for you.

There are edge cases like art or educational purposes, but using a fake ID to prove your identity to a third party is definitely fraudulent: you're saying "this authority attests that this is my identity" when the authority in question does nothing of the sort and the document claiming it does is forged.