Lets say the power supply is able to kill ripple dead, i.e there is a node at the power supply end; now take a typical 30cm cable and suppose that it can carry a wave at say half the speed of light, then one could have a half wave of frequency of 1 GHz
The problem now is that many components on the mother board are running at frequencies even higher than this, so one needs capacitors in the cable to short them. I suspect that normal power supply testers don't inject such frequencies on the end of the cable, and so the real benefits are not seen.
Just a thought...