Because they are hard-headed. They stopped using their freedom of choice in receiving the revelations of the Father Almighty and they did not come to the place where the Father can trust them as sons and daughters. That is why I am here, to directly answer your question: why can’t they produce sons and daughters? Because they used their freedom of choice to not obey the Father all the way.
But my choice was different. I obeyed Him all the way that is why I am here today.