Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe ? The usual approach of science of constructing a mathematical model cannot answer the questions of why there should be a universe for the model to describe. Why does the universe go to all the bother of existing?
It matters if you just don't give up.
If we find the answer to that, it would be the ultimate triumph of human reason--for then we would know the mind of God. (Closing words, referring to the question, "why it is that we and the universe exist.")
What I have done is to show that it is possible for the way the universe began to be determined by the laws of science. In that case, it would not be necessary to appeal to God to decide how the universe began. This doesn't prove that there is no God, only that God is not necessary.
God not only plays dice. He also sometimes throws the dice where they cannot be seen.