Because they have no objective meaning! Whether it’s an A or an 81%, the “grade” is still based on some teacher’s or test writer’s subjective judgment. In terms of learning, neither grades nor other quantitative measurements are valid indicators because learning, in it’s fullest meaning, can’t be measured. (See this article for the rationale behind that statement.) Learning can and is being assessed on a continuing basis in many learner-centered schools…and here, learning refers to the intellectual, social, emotional, etc. growth of each individual student. That kind of feedback is valuable to both the students and adults. Of what value is that letter or number except to create more unwarranted and destructive labels for students?