but why those Germans insist on capitalizing all nouns is a mystery to me

Just a convention. Why do we capitalize proper names and the first word in a sentence? There was a movement afoot in the 19th century in Germany to not capitalize nouns. I have a few linguistics books from then that do not and that looks funny! The English tended to capitalize many more of their nouns in the 18th century. Who knows? Maybe they were on to something that we've lost ...

Ceci n'est pas un seing.