One more dev-side reason for not extending the main map is the download size. A significant map update like this would require the entire main map to be re-downloaded, for every single player/owner of the game.
The map alone is the single biggest asset in the game and I wouldn’t be surprised if it easily shoots last 20, even 30gb for a download.
Forcing such a large download on millions of players, many of which may have slow or metered internet connections, is not ideal when the alternative is a smaller download of a separate environment.
For DiRT Rally 2.0 every patch was 10+ GB as locations/tracks got added or fixed, and despite the game improving as a whole the frequency and size of the downloads was a common bugbear from the community.