Samuel stands before the nation to deliver God's direct response to their demand for a king. He shares the complete prophetic message he received, which is not a practical list outlining the rights and duties of a future monarch. Those specific legal details are only established and written down later, during the actual lottery to select the king.
Instead, Samuel conveys a much deeper truth. He clarifies that their desire for a human ruler is not a rejection of his own personal leadership, but rather a rejection of God's direct leadership [אברבנאל]. By sharing this, Samuel proves to the people that God knows their hidden thoughts. He exposes the real motivation behind their demand: they truly want to abandon God's kingdom in order to be governed and judged by the same legal systems used by the surrounding nations [מלבי״ם].