It's kind of complex actually, the same happened with AA2 back then, I used to play it a lot for some time, it was fun, yet as I stated before, as a game it is a pretty simple one, with no complex mechanics, no complex gameplay, lack of features, etc..
But it's still fun for some people like me, despite the extreme lack of content lol
But yes, the game does lack in terms of content, same as with AA2. If a small team like Paralive's takes 10 years to deliver such a game, then developing a game like AA2 can be done in about 6 months (what a coincidence, that's what took them, way less than a year), expanding it can be done easily, because of the lack in terms of complexity (not talking about modding, modding is already hard just because of how the solution was compiled basically.. talking about the actual game itself). They probably never added anything new due to their focus on cash and multiple releases per year (this is not a 500+ employees company asking multiple studios to release different Call of Duty versions per year, it's one small studio developing as much different products as possible in the shortest amount of time, with almost no improvements over previous releases, Fifa and Pes but with nsfw
).
The other day I tried it, and yes it's a pretty fun game to play, but again, lacks a lot and you can "see everything" in a couple of hours hm Let's see if they someday make a proper sequel to AA2 but with actual gameplay changes that are significant enough to justify releasing a new game after like 10 years, rather than "hey, here's your 10 year anniversary koikatsu skin for AA2".