Really should take notes from eternum. It should all be together, instead of having to download and play an old version just to download and play the new version. It should all be one cohesive game so time isn't being wasted.
There are very real practical reasons why AVNs often do things like this, though. Downloading a 6 GB file can take hours, depending on your connection speed. If the first half of the game never changes, why waste those hours re-downloading it for every single version update? (Also, although this particular AVN doesn't do so, a lot of AVNs use Mega, which arbitrarily makes you wait several hours if the download size exceeds four or five GB.)