This is a really tiresome argument. Even if we didn't have a really good idea where morals come from, it would not be a proof of the existence of god. Every question doesn't have an answer. Every question doesn't have to have an answer.
Where do our moral values come from? This is a bit like asking where language comes from? We learn it from our parents. The process of learning is just not as transparent as with language. There is no spoken feedback that can be seen. Where do manners come from? We teach them to our children, first by making them do it without them understanding why, and eventually they internalize the importance of it. Morals are similar. We teach them by example, by punishing them for failing to comply to rules, and by telling them what we consider important. They learn by observing us, and by the things they read and see others do. Then we internalize what we learn. Just like language, like riding a bike, so that we cease to think about it consciously.
How did these morals develop in the first place? Easy. Those that worked aided survival; those systems that didn't died out, or could tolerate only a few adherents.
As for who we are accountable to? We are accountable to ourselves, to our peers, to our community and to the law. There is a range of punishments meted out in this life for non-compliance: from ostracism, to fines, to jail time, and in some societies, death. The system is not perfect, but that desire for perfect accountability, does not by itself, justify a belief in god. It is merely wishful thinking. Not being able to face the hard truth is not a "proof".