Because "America" is not just the United States of America, but the whole continent spanning from the northern Polar Circle down to Tierra del Fuego, close to the Antarctic circle.
Geographically, it is in North America (or America, the continent). Culturally, it is part of Latin America.
Central America.
Geographically, the Sierra Madre mountain range crisscrosses both Mexico and Central America.
Geographically Mexico is on the North American subcontinent (not to be confused with the United States, as it is usually known). Culturally speaking, Mexico is part of Latin America, as it shares many common traits such as language, religion an general traditions to other countries in Latin America, such as Cuba, Peru or Argentina.
America as a country is located north of Mexico and south of Canada.
Culturally, to Latin America. Geographically, to North America (the subcontinent spanning from Canada down to Panama).
Mexico is part of North America, but never a part of the United States of America
Mexico is part of North America.
Geographically, it is considered part of the North American subcontinent. Culturally, it is part of the Latin American region.
Central America - Guatemala, Belize, El Salvador, Honduras, Nicaragua, Costa Rica, Panama - connects Mexico to South America.
Mexico and Central America (Belize, Guatemala, Panama, El Salvador, Honduras, Nicaragua, Costa Rica) qualify as such. All of them are geographically, on the North American continent and belong to the Latin America cultural region.
America is geographically located just above the center of the western hemisphere. America is considered the United States. Canada is to the north and Mexico is to the south, followed by Central America and then South America.