"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.