The English-language name "Americans" for the English colonists in North America began to be used about 150 years before the creation of the United States in 1776.  By the time the United States declared its independence from England in 1776, "Americans" simply continued with the common usage of the pre-existing name.  

I presume that people in other countries do not call themselves "Americans" to describe their nationality, because they have voluntarily chosen to call themselves by another name.