Why do writers of the Bible blend God’s words i.e. “thus says the Lord” with human words such as speeches and human histories?
A good question. The Bible has several different genres of literature, depending on what God is trying to accomplish. Sometimes he wants to simply speak to us directly. Then he typically uses a prophet like Jeremiah or an inspired New Testament teacher like Paul in Romans. In that case he will say (or imply) “thus says the Lord.”
But sometimes God wants to communicate with us through revealing himself in a more visual way. So, he sent his Son and we can see God through the actions of Jesus in miracles and in the way he talked to people. In this case, God reveals himself through narrative. Genesis is primarily narrative and God speaks through these inspired narrative stories.
But sometimes God wants to communicate with us through how he acts in history. He wants to have us see him working in the world. So, he gave us the Book of Acts, or historical books like Joshua or 1,2 Samuel. Although these are human histories, they are still inspired by God, but in a different way.
But sometimes, God wants to communicate with us through direct commandment. Do this. Do not do that. So, he sends us laws such as in Leviticus or Deuteronomy (which, by the way, also has narrative. See point 2)
But sometimes God wants to communicate with us through poetry and feelings and the heart through us watching a person of God interacting with him. So, God gives us psalms.
It is interesting that in most of the Bible, it is God speaking to us, but in Psalms it is us speaking to God. In Job, Proverbs and Ecclesiastes, it is primarily us speaking to each other. Yet, even in these genres, God is speaking, only more indirectly.
Sometimes God speaks to us through illustrations, so he gives us parables.
But sometimes, God communicates with us through visions and images, so he gives us books like Zechariah and Revelation.
God even allows humans to speak, and to say untrue things in the Bible, such as in John 9:31 (an untrue statement by a somewhat spiritually ignorant man), or such as in Psalm 51:4-5 (where David, in his emotional anguish, says, falsely, that we only sin against God or that we were sinners before we were born).
God speaks to us in a variety of ways because we are complicated people, needing to be communicated with by a variety of means (Hebrews 1:1). He speaks directly, he speaks through prophets, he speaks through history, and he even speaks through the mistakes of human beings.