Because speakers of English arrived at the arbitrary decision that it is.

Whenever you're faced with the question: "why is x y?", you should ask yourself "is x y?". In this case, zero is plural... in English. But not in all languages! (I think in Arabic zero is singular.)

You can read about plural rules in different languages here[1]. For example some languages have three numbers: singular, dual, and plural. This is what Proto Indo European had and some descendants still do. Have you ever found it weird how "pants" or "glasses" are kinda plural but also kinda singular?

An interesting table to look at is here[2]. It compares all the rules in various languages for how to form cardinals. For example, English has two numbers: singular and plural and two rules to determine it: `n == 1`, `n != 1`.

My language, Romanian, also has only singular and plural, but we have three different categories: singular, plural without "of", plural with "of": `n == 1`, `n != 1 && n % 100 == 1..19`, `...the remaining cases...`. So we say "3319 horses", but "3320 of horses". It's very weird, but that's how languages work.

[1]: https://cldr.unicode.org/index/cldr-spec/plural-rules [2]: https://www.unicode.org/cldr/charts/46/supplemental/language...