Because it makes them money. It allows them to sell template-type programming that doesn't have to be coached at the individual level. If a coach with, say, 200 clients a month has to handle each client on an individual basis, reviewing every workout and assigning weights for the next workout to each client individually, that coach can't work the 200 clients a month. Hand them a way to assign their own numbers based on how things "feel" and you multiply the potential income from a much bigger client base several fold. That's all there is to it. They know it doesn't work, but it's the money. Some of these people are millionaires because of this, and it is a very viable business model, even if it is not a training model. It's the Planet Fitness of the personal training industry. You lose a few serious clients every month, who figure out that they haven't made any strength progress in a year, but they are easy to replace because the idea is so appealing to inexperienced people. And that's all there is to it.