Well, definitions are typically things people agreed upon, at one time or another. So what we have now, generally started from what everyone agreed upon during the antique times. Then, during the Renaissance times the definition expanded to cover all "four corners" of Earth. Australia was also included later, as time went by.

Physically Europe and Asia are a single continent, sure, and when discussing that, we use the name Eurasia. Funnily enough, Africa used to be considered a part of the Asian continent. In any case, definitions shift as the humanity learned more and we needed terms to discuss certain areas.

My point is: we aren't keeping definitions for historical reasons, even though there is history behind the terms. If we were to divide the globe in to continents for the first time ever, it would stand to reason the areas would be split similarly as they were, because geographically it still makes the most sense.

If we went just by tectonic plates or some similar way to determine continents, I guess we could get more specific easily. But in the context of language and understanding, combining south and north america to a single thing, or europe and asia, makes little sense.