Comment by jacobsimon

I can’t speak about other languages but in my head the simplest reason is that English “a/an” specifies “one”, eg “I have a cat and two dogs.” I googled and confirmed a/an is Old English for “one” but otherwise they didn’t use articles. I assume it’s a similar origin story for other languages like “un/una/une”. In that case, it’s equally strange that other languages would make “one” plural imo.