I’m guessing you’ve never read the constitution.
Article III of the Constitution establishes and empowers the judicial branch of the national government. The very first sentence of Article III says: “The judicial power of the United States, shall be vested in one Supreme Court, and in such inferior courts as the Congress may from time to time ordain and establish.” So the Constitution itself says that we will have a Supreme Court, and that this Court is separate from both the legislature (Congress) and the executive (the President).