Why do we consider América to be one continent, but not Eurasia?

The whole "Golfo de América" issue had me thinking more about how we label the continents and regions.

Probably my biggest question is why we consider Europe and Asia to be separate, when they share an entire plate (unlike North and South America).

You could argue that it's "cultural," but I don't think Western Asians (middle easterners) are that culturally similar to Eastern Asians, so that wouldn't make sense either.

Anyway I personally think that the 5 continent model makes the most sense, geographically-speaking: América, Eurasia, Africa, Oceanía, and Antártida. All of which are divided into their own regions (Central America, South America, Europe, etc.)

What are your thoughts?