The continental distinctions as they are is pretty much geopolitics. There's no passable land route between North and South America, because the connection is

The continental distinctions as they are is pretty much geopolitics. There's no passable land route between North and South America, because the connection is through thick jungle. Similarly, pretty much nobody wants to go through the Ural mountains to go overland from Europe to Asia. Since little overland commerce or credible overland invasion routes exist between continents, they can be thought of mostly independently.