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.
> In that case, it’s equally strange that other languages would make “one” plural imo.
Why? It's very common in English:
I don't want those ones.
In this case the number has been repurposed as a placeholder noun, and the noun is what's pluralized, but that's also what happens when the number is repurposed as a determiner and the determiner gets pluralized.
I think it probably just boils down to whether the word is functionally necessary to understand the meaning of the sentence and how the language evolved...
For example, in French you don't usually pronounce the "s" at the end of a plural noun and lots of singular nouns end in "s", so the articles become necessary for determining if you mean plural or singular. In English you pronounce the "s" clearly and very few singular words end in "s", so you just don't need the article to distinguish anything, it becomes functionally redundant.
This is not a good explanation, because you could just as well say "I have cat and two dogs". The fact remains that "a cat", "cat", and "the cat" (and "one cat") all have separate meanings; while for plural, you only have "cats" and "the cats".
There are some nouns that are always plurals e.g. "scissors" or "pants", and those things still arguably need indefinite/definite distinction, right? Actually no, the very use of plain "scissors" instead of "*a scissors" demonstrates very well the redundancy of the indefinite article, but let's pretend.
Again, e.g. Bulgarian uses едни ("ones") as essentially plural indefinite article, and for genuinely plural things too. It's not nearly as pervasive as the English usage (the null article is used in most cases), but it exists. While in English you could reply to "Where are you going?" with "To hang out with some friends" ('it doesn't really matter to you which ones exactly'), in Bulgarian that "some" would be translated as "едни", not "няколко" or "някои" (technically, you could use that last one, but the meaning would be that you're yourself not sure with whom exactly you're going to hang out there).
I can't answer the why question, but out of all Germanic languages, only Faroese has retained the plural indefinite article, although its use is restricted to nouns that 1) only occur in the plural and 2) indicate «a pair/set of», e.g.:
einar buksur (feminine plural) – a pair of pants
einir skógvar (masculine plural) – a pair of shoes
Other than that, Germanic languages typically do not have a plural form of the indefinite article.
But in "The doctors are female" you do because the subject is some specific group of doctors. It's singular because its a single group of things.