Much2learn:
I think ECs show another aspect of a candidate: can this person manage their time well? Or put in another way, together with a person’s class schedule and grades, they want to see if the person take a rigorous academic schedule and do well without spending all their time just on academics.
Schools do not want to encourage the following behavior: take as many AP credits as possible each year, maintain all As, prepare for SAT/ACT from 8th grade, and nothing else matters.