How much reps are we talking about and what exercises are they doing them with?
Some exercises do not lend to high reps and some would be purposeless to do with low reps (e.g., high pulls, lateral raises, etc.).
Many advanced lifters are at a point in which they are just maintaining the size they have because they are peaked out, even if drug aided.
Some exercises might even be done unsafely with high reps by noobs. For example, squats, power cleans, deadlifts, overhead presses, because with a lack of strength and a sturdy body with a foundation of muscle, form can break down in the beginner, whereas someone who has developed very good coordination and a good sense of body awareness can handle money exercises for high reps with good form. It can sometimes be actually easier to teach someone the money exercises with low reps in the beginning.
I think I've done OK for myself body-wise (perhaps not great), and I do 6 to 15 reps for exercises.