Or do they and I just haven't seen it?
If I was a transwoman with a penis my thoughts would go something like this.
'I don't want to use men's facilities because I don't feel like a man. I understand that some people feel that penises don't belong in the women's. But a unisex facility would mean I could use it and feel like a woman, and nobody who thought I was a man would be bothered I was in there'. Win win, surely?