There are a number of ways to respond to this.
First, maybe we haven’t! There is one school of thought which posits that free-will is an illusion.
Second, quantum physics doesn’t hold that physics is strictly deterministic. However, I don’t know that randomness at the sub-atomic level has been shown to scale up.
Third, there is a theory that free-will could be some kind of emergent property. For example, water has properties that that neither hydrogen nor oxygen alone have, and a brain has properties (free-will) that its components do not have.