Why do RCDs / GFCIs sense differentially rather than directly?
I feel the GFCI (Ground Fault Circuit Interrupter) is slightly mis-named, as it is intended to trip if any current leaks in or out of the circuit, regardless of the path it takes.

If there is a fault, the "missing" current need not flow through the ground wire that goes by the GFCI - it might flow from a kettle, through you, to the water pipe, and then to ground, with no current flowing through the ground pin in the outlet.