Confusion with RCD principle of operation
As Martin hinted in his comment, an RCD is most usefull in a situation where a fault is likely to cause a current flow to ground/earth. This is actually a very probable situation for a fault: touching one wire (while being somewhat ground-connected) is much more likely than touching two wires simultaneously. And it doesn't need to be you: the compromised live wire could touch a grounded part of the appliance.

In my country (230V, both wires live) there are generally two classes of appliances: metal casing earthed, and insluated casing 'double isolated'. In this situation a RCD is very usefull for detecting a compromised wire touching a metal part of an earthed appliance.