Why does the fitness industry ( well mainstream) support the myth of high reps and low weights?

For five years I have never put on muscles since trainers would instruct me to lift weights 20x times and I never put on any muscle. I started training with a few power lifters and I've quickly gained a muscular definition in 6months time. Does the fitness industry use this myth as a way to keep people in plateau limbo?Why does the fitness industry ( well mainstream) support the myth of high reps and low weights?Trainers who were telling you you'd gain significant muscle with high reps and low weights are quite simply inept and should find a new career path.Why does the fitness industry ( well mainstream) support the myth of high reps and low weights?well high reps and low weights give volume, not strength.