Perhaps because marriage annulments have nothing to do with what happens after the marriage begins. They are concerned with determining whether or not the marriage was valid at the time it occurred. They are fundamentally different from a civil divorce, most people's perceptions notwithstanding.
Why can we not allow that once healthy and valid marriages can die for reasons other than the physical death of one or the other. Baptism and Orders are permanent - where does it say marriage is. The physical death of one party will end the marriage, though some would like to hold that it continues in heaven. A marriage can morally die. Slaves separated from their spouses were allowed to marry another. Leave marriage to the consciences of the people, not to the control of ecclesiastical tribunals.