It's not only in Germany, it's an EU mandated thing to have e-invoicing to prevent fraud and whatever other things the bureaucrats in Brussels felt like it would be solved with technology.
And yes, sadly the powers that be decided that this crap needs to be XML. Because why not, why use a modern standard...
What about XML isn't modern? It's a far more capable format than JSON or anything else you can devise.
How about we agree to disagree? XML has no place in the modern world. If it wouldn't be for the likes of Microsoft, IBM, Oracle and a few others that keep using it and pushing it, it would have sailed into the sunset a long time ago.