Exactly, and agreed. Unfortunately, it is very difficult (read: impossible) to find 2D artists who would do this sort of work for basically free of charge. IDK, I'm not sure if they are paying their staff or not since the game is basically free & relies on donations instead of paywall system, but one way or another they need to find someone passionate & devoted to drawing who would be willing to do this for the amount they (devs) can afford.
Until then, Amity Park will be on hold, and we probably won't be seeing another update for at least a year or so. Actually, it's already been one year between that last update & the one before, so it is only safe to assume that the next update won't come out until some time in 2024.
Edit
Thought I'd point this one out, even though it is already pretty much obvious to everyone. The reason why most RenPy games look alike is because they all share the same (or similar) assets. Locations, characters, etc ... it is all obtained from the same source and that is what most of these adult games do. Sometimes the author will simply change character's hair style, or facial expression but in most case scenarios they will look more or less the same.
And this is a huge problem for Amity Park, since the entire game is unique, uses unique drawing style & animations. You cannot simply take the school for example or the park from another game & use it for Amity Park as it wouldn't make any sense. Which is why they need to make their own, and this takes a LOT of time. It is basically the entire game made from scratch, using all original art & drawings that's inspired by Danny Phantom cartoon.