Why is a partial thyroidectomy not a cure for hyperthyroidism?

Pituitary regulation: The thyroid gland is regulated by the pituitary gland, which secretes thyroid stimulating hormone. A spontaneously overproducing thyroid may continue to grow and secrete if not removed completely or suppressed medically.

Remnant is bad: Most common hyperthyroidism is grave's disease. You need to do total or subtotal removal because the remnant can still cause problems. Surgeons used to try to remove "just the right amount" to hopefully not cause hypothyroidism. There are risks of leaving to much and then not helping the problem and a high risk of hypothyroidism anyway.