No USA has hijacked that. What you are talking about is the Americas plural with an S, which include north, south and central America.

I absolutely agree with you that it is stupid and arrogant of USA to call itself America. But it is shorthand for United States of America.

In my own language Danish “Amerika” is indeed the 3 Americas combined, but in English “America” is used interchangeably with USA.

https://en.wikipedia.org/wiki/United_States

The United States of America (USA), also known as the United States (U.S.) or America,

https://da.wikipedia.org/wiki/Amerika

Amerika er den samlede landmasse indeholdende Nordamerika (inklusive Mellemamerika) og Sydamerika

Translation: “Amerika” is the combined landmass containing North America (including Central America) and South America.

It depends on the language, and here we use English. And yes I agree with you it is stupid. But people from USA are called Americans, hence it is sometimes more logical to use America than USA.

https://en.wikipedia.org/wiki/American

American, something of, from, or related to the United States of America, commonly known as the “United States” or “America”

I don’t know what your native language is, but you are probably transferring norms from your own language to English, that in this context makes you simply wrong.