I had no idea that Ghostbusters: Afterlife was actually III. This is a failure of marketing, especially after the stink of 2016. I would have gone and saw Afterlife if I knew it was a sequel to the original 2 films from the 1980's. F for Harold Ramis, who didn't live to make the third film as he died back in 2014.
As 2016 is a reboot and separate continuity from I, II, and Afterlife (III), it makes sense that isn't included in this set. The fact that it was a shit film only made the decision to exclude it easier.