Because grades should not be viewed as rewards, but rather as an indicator that learning has taken place, both by students and teachers. What you propose is so

Because grades should not be viewed as rewards, but rather as an indicator that learning has taken place, both by students and teachers. What you propose is something of a Goodhart's Law violation: "When a measure becomes a target, it ceases to be a good measure" Do we want an economy of grades, with all the unintended incentives that would give everyone participating?