Why would God say he was rejected as the king, if he included instructions for a king (Deut. 17:14-20)?

God says, “They have not rejected you, but they have rejected Me from being king over them. ⁸ Like all the deeds which they have done since the day that I brought them up from Egypt even to this day—in that they have forsaken Me and served other gods—so they are doing to you also” (1 Sam. 8:7-8). Why would God say that desiring a king was sinful, if he included instructions for the king in Deuteronomy 17:14-20?

The people wanted a human king for the wrong reasons. They wanted to be like the other nations; see verse 5 (“Now appoint a king for us to judge us like all the nations”). They wanted to trust in a visible and finite leader, rather than the invisible and infinite God. This is why Samuel reminds them that God was the one who rescued them from Egypt—not a human person (v.19). Moreover, they didn’t ask God for a king; they asked Samuel for a king (v.5). Deuteronomy 17:15 explains that God would be the one to appoint a king; however, the people wanted to choose their own king. While God also chose Saul (1 Sam 9:16; 10:1, 24; 12:13), the people were choosing him for the wrong reasons (e.g. stature, good looks, etc.).