In the case of MapleStory, in the past much of the game logic used to happen in the client side, that's why it was easier to reverse it (And create cheats for it).
In many other games that have private servers: If there was no codebase leak, basically guessing what you can't know. That's why many private servers doesn't work 1:1 when compared to the official servers, people just tried to make the calculations as close as possible after reversing it.
GTA 5 has private servers as well, and it has a similar fat client architecture. It also uses P2P for a lot of things which was probably helpful for the FiveM developers.
MapleStory was also kind of an interesting case because the developers were careless enough to include debug symbols with their binaries on multiple occasions.
In many other games that have private servers: If there was no codebase leak, basically guessing what you can't know. That's why many private servers doesn't work 1:1 when compared to the official servers, people just tried to make the calculations as close as possible after reversing it.