When I've thought about this possibility in terms of how I would do it, the reason that came to mind was, "because that way you don't have to slow down the turns by simulating everything for the areas that aren't discoverable yet." Since, as you mention, otherwise map expansion over eras can be figuratively achieved in existing iterations - if nothing else, by preventing travel across oceans until a certain technology is discovered.
The CPU cycles saved are one thing, but there is also the outcome of those simulations. The AI running unchecked on a continent might produce silly things so that the new world is not fun to engage with. The terra maps always had the tendency to be overrun with barbarians, because noone was there to keep them in check. And you could not really change that without changing the balance on the starting continent as well. Or the minor civs could have killed each other and razed each others cities, so there is only one city state left.
By expanding the map, they can start with a blank slate and fill it with a predictable amount of stuff they want the player to encounter.