Same reason why people from the United Kingdom are called ‘British’, despite Northern Ireland not being in Britain.
There just aren’t better proper adjectives for these 2 countries.
While you can say ‘North American’ to mean anyone from North America and not specifically the US, I’m not sure there’s a fitting word that refers to anyone from North or South America. Although, at that point, the group of people you are talking about is perhaps too broad to be useful in most cases.
Same reason why people from the United Kingdom are called ‘British’, despite Northern Ireland not being in Britain.
There just aren’t better proper adjectives for these 2 countries.
While you can say ‘North American’ to mean anyone from North America and not specifically the US, I’m not sure there’s a fitting word that refers to anyone from North or South America. Although, at that point, the group of people you are talking about is perhaps too broad to be useful in most cases.