Despite not asking for an explanation, I’ll give one anyway since you seem not to have resolved your grievance.

“American” is the correct adjective in English to describe the United States’ people and government. There is simply no equivalent to the Spanish “estadounidense.”

Furthermore, North America and South America are considered to be separate continents, and if you want to refer to them both together, you say “the Americas”, plural.

https://en.m.wikipedia.org/wiki/Americas