I have a conspiracy theory. When you boil down to it, who's really responsible for pushing the once a week frequency? Muscle magazines. But they only really started doing it during the '80s. You had a few big big name BB's (Tom Platz) throwing in behind this ultra-intensity high volume, once-a-week stuff. Coincidentally, that's when the supplement industry started becoming really big.
So basically that was the introductory period to the current state of bodybuilding: the masses accepting advice from steroided pro's for the purpose of selling supplements.
That's all well and good, but then realize this: from a business perspective, supplement companies DON'T WANT PEOPLE TO GROW. The sooner you reach your body goals is the sooner you STOP buying their mass gain supplements. Protein, creatine, prohormones, mass gainer powders, all of which are intended for adding LBM, probably comprise the vast majority of supplement sales.
So my personal theory is that whoever runs these magazines knows that their plans don't work all that well, but they do it intentionally to perpetuate supplement sales. It's the only way I can rationalize the vast dissemination of misinformation.