The answer to this and to most questions about God, is found in God's love for his creation. God yearns not for perfection, but for relationship. So instead of destroying creation, he wipes it clean -- but saves Noah and an arkload of others.
There's an important lesson to learn here about how God works. God does not delight in destroying anyone or anything. God doesn't wait for you to mess up and then smack you. Rather, God is constantly on the lookout, even in the middle of brokenness and wrongdoing, for things he can use to bring about good. So your stubborn streak, brought through God's redemptive processes, becomes the root of your unwillingness to turn away from your faith. Your suffering through difficult times opens a compassionate place in your heart, and God uses this compassion to extend comfort and care to others who are suffering. God is always looking for something to redeem. So Noah becomes the symbol -- not of things in your life that are perfect already, but things in your life that give God raw material to work. Noah is not that great on his own -- we'll see this after the flood -- but he's willing to be obedient when God tells him to build an ark.
It's less about perfection and more about availability.
One of the devil's favorite tricks is that he gets us to believe we have to make ourselves good enough for God. So we hold ourselves back, trying to live better, be better, when all God wants is really what we've just taken from him -- the possibility of relationship. It is in the middle of the relationship that God cleans us up and heals our hurts. Outside the relationship, we are hopelessly on our own.