I you are lying down on the ground fully extended you risk the potential that the voltage (extremely high) will enter at one end and exit to the ground at the other end. That extent of body has at least some resistance and that high of a voltage differential created between the ends could generate enough current to kill you dead.
The “couch” is to present the least voltage potential difference with respect to resistance on the ground - smaller potential difference, less ultimate current.
Sounds like step voltage
Agreed - the couch is the safest position. But I always unplug my expensive TV during a big storm!