We humans have a sinful nature. It’s not just what we do, it’s who we are, and we can’t do anything about it ourselves. Sometimes to cope with our inability to do anything about it, we attempt to deny our sinful human nature. But God Himself doesn’t deny our nature, that’s why he sent Jesus to pay for our sinful nature, and why He gives us a new one over time.
But why doesn’t God just give us a new godly nature all at once? Because that would remove our volition and participation in it. It would remove the personalized and loving way that God deals with each of us. God does not seek to quash our individuality or to usurp our volition; He gave us these things in the first place. God wants our individuality and volition to be used to and for the best ways and purposes. God is not a god of assembly lines and cookie cutters. God personalizes the way He gives us each His new nature.
More on man’s volition and God’s sovereignty:
Love and Volition
Sovereignty of God and Volition of Man
Volition and Personal Responsibility