Because there are a litany of counterexamples to the myth that earnings equate to economic value. Teachers, police officers, and social workers immediately come to mind.
That sounds like a great argument for subsidizing teacher and social worker education instead of the wholesale forgiveness of student debt which includes people with degrees in poetry and pottery. Poetry and pottery are both valuable to society, but honestly a lot of the people who are good at those things don’t have matching degrees. Also, forgiving loans for pottery degrees competes with our ability to subsidize education for teachers.
The obvious solution is to subsidize teacher pay etc, not to subsidize education to be teacher! If teachers are well paid, you wouldn't need to subsidize teacher education.