This can also cause delivery failures the other way around - a friend in Germany once sent me a package but didn't bother writing the apartment number on it because she assumed the postman would use my name to find the right box. Instead it got sent right back to Germany. (Austrian bureaucracy is just as unforgiving as German, they just have different rules to follow...)
They do (at least sometimes?). They are just not listed anywhere and barely used. I have my number in my contract, and e.g. Vatenfall have it to connect the meter number to the flat number.