"America" as a single entity, devoid of qualifiers, is the entirety of North and South America. A massive and diverse swath of Earth with very little in common. I don't see how that's a useful definition, and most English-speakers have agreed so "America" instead is shorthand for the giant country with "America" in its name.

We use descriptors because they help. "The Americas" is the whole thing, "North America" is everything from Panama to Canada plus some distant colony islands because this is politics not tectonics.

There is only one country in the Americas that has America in the title. Everyone is aware of the continents, but only pompous douchers bring up the "technically, Canada and Mexico are American too" nonsense.