The belief is obviously common, even if the experience is not. It may have to do with the way we train lifters and the way these highly successful PTs who train competitive lifters program them. I'll bet if you ask them how many times a week they train the lifts, they'll tell you 1x/week. If your lifters detrain between workouts, they might report DOMS, and they might not. If your "competitive powerlifter" program features constant variation in assistance exercises, you'll be sore. And if the person taking the report wants you to be sore, maybe you're inclined to say you're sore, or even believe you are sore. At any rate, the vast majority of people who are effectively training a program that relies on gradually increased loading, or even volume, and does not rely on exercise variation as a training variable do not get sore every week. We have the data.