Sorry for going off topic here, but why do people call inhabitants of the United States of America for Americans, while America is the landmass from Canada in north to Chile in south. Shouldn't all people living in America be called Americans, instead of only 33 % them?
But anyway: happy birthday to all USA'ns out there.
This is not just some new thing that happened. The region that the United States now in habits has long been known as 'America', before even colonies were established by the British.
It just goes to serve then that those living in this new world called 'America', the reason for which name is quite a story on itself, plenty of controversy there as well, then that they should be called 'Americans'.
The notion that America, and Canada, and Mexico, and the lands further south are part of these American continents of North and South comes much later, and you are really comparing apples and oranges for these terms come from different time and places.
U.S. citizens are Americans because they live in the nation of the rebellious colonies, where Canada is Canada, never broke away or succumbed to U.S. invasion, not that the we didn't try back in the late 1700s and early 1800s to take Canada.
So to say that everyone living in North and South America are 'Americans' requires quite a bit of abandonment of tradition and history, and one I don't think is really worth doing.
After all, it is the United States of America, and our neighbors to the south are the United States of Mexico. And Canada....well.....whatever the deal is with Canada I don't really care.