You did not know that large parts of the world default to 12h time?
This is absolutely NOT an American thing. I am not American and have never lived there. I am 58 years old, British and Irish, and lived a decade in Nigeria and a decade in Czechia. I have travelled widely around Europe and can get by in 6 foreign languages.
12h time in real daily life is more common everywhere I have ever been. Only minor subsections of humanity -- military people, maybe -- work on the 24h clock.
That is the direct opposite of my personal experience.
I do not live in the UK; I live in the Isle of Man, nearby. I visit for work often, though. Yes, the 24h clock is used for things like transport times, but in daily life, written and spoken, it is basically always 12h. I would estimate 95-99% of the time.