What's the primary key to identify a German citizien? The full name is certainly not unique. Even combined with birthdate duplicates do probably exist. Some countries have something like a social security number. Of course those exist in Germany, too, but using them cross-functionally has long been deemed unconstitutional. So the tax office can use the tax number, but that cannot be shown e.g. in your passport or drivers licence. Analogously with every other identifier. I think some law and order politicians have tried to weaken this up, but I guess it's still not fully allowed.

Asking out of real curiousity. I am German citizen, but have not lived there after the first government office got a PC.