[silentbicycle]

It doesn't. It is a wild exaggeration. It usually isn't the 300+ variables, just a few obscured by multiple levels of indirection that can cause unexpected inconsistencies. (It gets even worse when you drag in inheritance...)

Each function only depends on the arguments passed in, and variables outside of its scope which are referenced. However, the same is true of every function called inside the function, and the dependency is transitive, so the impact of changes to variables that aren't explicitly threaded through leaks outside to functions that never mention them. That's the problem. Hidden state.

It's also the kind of issue that usually looks incredibly contrived in small (say blog-post-sized) code samples, but isn't funny anymore when you have to deal with big lumps of spaghetti code. Global variables make reasoning about dataflow hard. Real problem, poor description.