Did anyone bother to contact the original developer and ask their permission to make alterations to their game? Does the game have any terms and conditions attached to it? Just because a developer doesn't include any terms and conditions to their game, does not necessarily mean they do not have them, if they did not they would have to be insane. Then there is the possible issue of copyright infringement. You cannot just go into someone's game code and alter it without their permission, it would be tied with their terms and conditions it would be in their terms and conditions, prohibiting reverse engineering etc. Just these few things alone, are enough to put anyone off even bothering even with abandoned games, the copyright still belongs to the original developer unless they open source it.
As for the game assets, I don't see any problems there, given people can just copy the assets as you said.
Unless of course there is some sort of agreement in place while something is in development that only the designated people can add, alter or completely change the code..if it became abandoned it would literally become open sourced (but not always, the dev, creator still retains the copyright unless it does become become open sourced, regardless, open source can still retain different subsets or types of copyright owned by the dev, creator, but that is neither here nor there). Many vn's are like this, and many of them show a lot of promise but get abandoned either before their first update or just after, because the Dev either doesn't have enough money, cant get any patrons to help him or her, or they realize just how much work is required to create a vn in the first place, and either didnt realize just how time consuming creating an adult vn is and don't have the time or interest to continue development or they are just starting out and trying to find their feet by experimenting with the tools and assets they have.
Unfortunately in this case, unless permission is given to alter and or change, add new code, renders, animation, it is very unlikely to ever even be touched in the first place, just not worth it in the end.