There isn't any other thing to assume, that's just the way it is on our side, maybe not on the Japanese side, but on ours for sure. This explains the low demand and subsequent low supply for mods that actually deal with the game. Notice how 99% of the mods are made to work specifically with the studio, there are actually only a few that were specifically intended for the game and the others that do have some function within it simply happen to do something both the game and the studio can do. Then you go on the Discord, which comprises the majority of modders on the English speaking side and the vast majority attitude there is very much in favor of studio over game, they've openly admitted as much.
As for why this attitude of studio over game took hold, you can blame Illusion listening to the Japanese customers, the majority of which wanted a customization focus, as if they were in an echo chamber and not balancing everything with a middle ground approach for that one as the customization focus took away development time for the gameplay.
Unfortunately, the lack of similarities in the code are why things like mods can't be used as they are from the original Koikatsu. There are certainly similarities, ones specifically built in to allow using characters and other things the player creates, but those are few. It was specifically designed not to be a whole different game specifically for that reason, a whole different game would be like trying to bring cards from Artificial Academy 2 into Koikatsu Sunshine, they were never designed to work together, but Koikatsu and Koikatsu Sunshine were even if to a lesser extent than AI Shoujo and Honey Select 2 were.
The stigma is most definitely not the same as the west, I would barely call what the west has a stigma since at least we don't make producers censor it beyond recognition even if some people don't like seeing it. It does sell in both cases, though. The stigma is not the sole reason, but it is part of the issue. The stigma makes people not want to keep working with Illusion once they have enough experience to go mainstream, leading to almost constant rotation of new blood on what could very easily be a per game basis at this point and makes them hide even working for Illusion from their own families to avoid the shame it would supposedly bring. The other reason why the game isn't bigger I already mentioned, they listened to their customers and went all in on the customers' desire for customization, spending way too much time on it. This left too little time to flesh out the game's actual gameplay, the element that makes it a game in the first place. This is a pattern that has been repeated ever since Artificial Academy 2 was fully released with all DLC, the games have slowly been going downhill ever since with less gameplay each time minus the exception of the Honey Select series always having had minimal gameplay by design.
