Possibly because German nouns are always capitalized
and it is just copied that way?

While this is true today, there was a period in the 19th century when the Grimm Brothers and others stopped capitalizing all nouns and followed rules closer to those of English capitalization. You can see it in the Grimm German dictionary (sort of equivalent to the OED in size at least). It did not catch on.

Ceci n'est pas un seing.