Because it's part of our nature, not anything to do with choice. You can't make yourself invisible. Could I complain, then, that you don't have free will? Of course not. One immutable aspect of being human is that you have material substance not subject to invisibility. It has nothing to do with free will. Could you complain that you have material substance against your will? Not convincingly. Could you complain that you were born of your parents against your free will? Some guy tried that in court not too long ago (
https://www.bbc.com/news/world-asia-india-47154287): giving birth to him without his prior approval. It's nonsense, and the man can't find a lawyer to take up his case. I understand why.
So also, your being born in sin was not a violation of your free will. It has nothing to do with it any more than your being born a human rather than a giraffe was an affront to your free will.