What do you mean by "because they require a lot of detail"? I assumed that most classic RPGs are an engine reading tiles and some intermediate scripting/config language for the behaviors.
They are also usually heavier on content than other games so you have file format shenanigans like custom compression schemes or crazy memory management (see Ultima 7) to make them work / get to their internals.
If you’re lucky they have a scripting VM that drops much of the gameplay in place (Adventures of Robin Hood for example) because you can just translate it, but many older games are a mess of data files, data embedded in the EXE and procedural rules.
Coming from BioWare games and having contributed to making that massive complexity mess testable in the 2010:, RPGs are the hardest to test games overall because branch converge is very hard to achieve.
If you take for example SWTOR - designers store quest variables on random animals in the world. Thoughts and prayers even for the LLM ;)