I always explained this difference between floor numbers in my country and the US by language: in my language the word used for upper floors only means upper floors, so the 1st floor has to be above the ground floor; while in English they're all floors, so ground floor is the first floor.
But I didn't know the British use the same system as my country (and most of Europe afaik). They could've just adopted the same system, despite language, for consistency.
Are you ever zeroeth in line? What’s the zeroeth thing you do after waking up in the morning? Do you ever launch an argument with “Zeroeth of all…”? Do you remember your child’s zeroeth words or the time they took their zeroeth step or their zeroeth day of school?
That makes sense if you are counting but i prefer a description. If you are in a building with only one floor, you are at the first floor. There is no such thing as the zeroth floor. Because it is the first floor you see and grasp its existence. Its the first thing you see. There is no such thing as the zeroth. Zero implies nonexistence. You cannot use it when you are counting things that exist. Another example: ln a race there is first, second and third. Who would the zeroth refer to? It would have to refer to the last person who has crossed the line without participating in the race. He might have crossed the line before the first but he does not exist in the race so he does not get a reward.
The word for floors in my native language is the same as the word for "upstairs". And it's the same in a lot of European languages (French étage). And the ground floor has its own separate word. Using French as an example again, the word is parterre, literally meaning "on the ground". So numbering in Europe goes ground floor, first floor, and so on. Now in English, the word being the same, it can sound confusing, but I assume the British just adopted the same system as the rest of Europe for consistency (although they're not usually known for doing that).
Floor 0 is for those weird buildings built on uneven ground where you enter floor 1 from one side, but floor 0 from another, so it's neither really underground to warrant negative floor number, nor is it fully on the ground to be positive.