• Well, it’s just how math and numbers in English work.

    Cardinal numbers, the number of things you have, start with zero because you can have none of something (or less with negatives, but that’s neither here nor there).

    Original numbers, Numbers that show which things were in what order (first, second, etc) start at one, because you can’t really have a zeroth something because then it would really be the first one.

    So year 1 is 1 because it’s the first year, and it starts the first century. It would have been entirely possible for English to make the names a little nicer, but given that it isn’t, the math means the first set of one hundred years are the years before the one-hundredth year and cetera.

    • I mean, zeroth would still be zeroth; it’s just based on the cardinal the moment before it arrived rather than after, assuming you start with nothing and add objects. Unfortunately that’s not conventional, probably in any language, and so you get a situation where a positional notation clashes with how we want to talk about the larger divisions of it casually. This sort of thing is exactly why computer science does use zero indexing.

      Relatedly, there was also no year 0; it goes straight from 1 BC to 1 AD.