Of course, SCOTUS is assigned the judicial job of properly applying the Constitution to the facts of a particular case. No problem there.
However, ALL legislative powers are vested in Congress by the Constitution. The Constitution vests NO legislative powers to SCOTUS, only judicial power to decide individual cases and controversies that reaches to the parties of the case and any other case with similar questions of law and fact (”precedent)”.
If rules and laws were applied in the manner that you insist, there would be little consistency because cases could be decided differently even when the material facts were similar. At their best, judges are consistent, applying the law to similar effect in similar cases. At their worst, judges decide cases based on who they want to win, which is the way most people view court cases.