This is another one of the weird American-isms that many Americans don't realise isn't normal everywhere else.

Boris Johnson was born in New York. "He wasn't born in this country" probably wasn't even on anybody's top-100 problems with Boris as Prime Minister.

The US president is both Head of State and Head of Government. It turns out there's a bunch of countries that require the head of state to be a natural born citizen: https://en.wikipedia.org/wiki/List_of_presidential_qualifica...

The requirements to be US President isn't to be born in the US, but to be a "natural born citizen."

While the rules of being a natural born citizen is more complicated if born outside of the US, you can generally become one if one of your parents is a US citizen.

I wouldn't call it an Americanism, per se. There are plenty of countries where you can't become a citizen at all without having a relative who is one. There are also plenty of countries where, even being born there is not sufficient for citizenship (in fact, only 35 countries in the world grant citizenship unconditionally via being born within the borders).