Because deciding what laws "should be" is a job for legislators. Judges are supposed to decide what the laws ARE.
"It is emphatically the province and duty of the judicial department to say what the law is. Those who apply the rule to particular cases, must of necessity expound and interpret that rule." —Chief Justice John Marshall, for the Supreme Court in Mardison v. Marbury.
Ironic considering Marshall was basically making up laws and granting himself powers that were never explicitly granted to SCOTUS by the Constitution or any other legislation in that case.
