Because it is baked into the class itself. Ruby is global variables "fiesta", because all classes and modules (and functions that begin with first uppercase letter) and became global once loaded.

Because of that, you can not have 2 parallel versions on the same lib, in the sane app, not possible. You load classes once, and they are accessible everywhere, magic! No need for loader headers as in JS, Python, Java, Go, ...

That said, ruby devs like to have "sane" extensions like to_json. People proffer to write "@object.to_h.to_json" instead of "JSON.parse(@object.to_h)" and not to think about loading json lib. I agree that Rails is abusing this too much.