Canada - Canadians, Mexico - Mexicans, Brazil - Brazilians.
United States - ......United Statesians?
That's like people from the DRC calling themselves 'Democratic Republic of the Congolesians.' Nope, they just shortened it to Congolese. The United States of America just did the same thing. 'Americans.'
They all use it, and for the most part so do their neighbors.
Until some clever soul figures out a better way for the bologna of the North American sandwich to refer to it's citizens, the largest collection of English speakers on the planet will probably keep on trucking with the same boring term.