IDK why some games save locally,[...]
Because it's the default behavior of Ren'py since versions/years and implied by the compatibility with previous versions of Windows and probably also by the compatibility with the other supported OSes.
but I'd say the majority are saved in the global RenPy save directory.
No. Even now that he double the save by a copy in
appdata, Ren'py still see
game/save as main save directory. When loading, Ren'py give the priority to
game/save,
appdata being just a kindness copy for Windows users, letting them conserve their save files even after they delete the game.
Therefore, editing saved files directly in
appdata would make the edition useless unless you double it by the deletion of the saved file stored in
game/save.
Find a .net developer not knowing this hard to believe, but I have come across many that haven't so I never assume..
You know that Ren'py have absolutely nothing to do with .net, and that, because of this, the behavior of .net have absolutely nothing to do here, right ? So, it's a good thing that you never assume, else you would have asked for something which goes against Ren'py behavior.