That wasn't the point of the movie.
I saw Children of Men yesterday. I'd heard criticism of the movie because it didn't tell the viewer why there were no children and why the infertility rate was 100%.
But the why of it doesn't matter -- it was making a point. A point about how humanity in general views children in this world. The world in general would appear, given current standards of living worldwide, to view children as disposable -- a nuisance even.
They are the first to succumb to starvation and disease, lack access to medical care, are ignored casualties of war, are sold into slavery, are abused, and if all that doesn't get them, natural disasters will. Children aren't generally able to protect themselves -- that's what adults are for.
As a collective, humans (and I include myself in this), while we may be outraged when we hear about the massive deaths of children, may even protest the horrendous situations in which children live -- well, very little ever really happens in response. So we throw up our hands in despair, and children still suffer.
What the movie is showing us is one view of what life could be like if we continue to place such little value on the lives of our children.
It wasn't a coincidence that in the movie, a child hadn't been born in 18 years. Think about it -- 18 years, the time from birth to adulthood in many societies.
And the world had to be without new births for that long, to learn to rejoice in the first birth in 18 years. For the world to finally see the importance of the children that we previously took for granted, to treasure them for what they are -- our future. For without those children, humankind would eventually cease to exist.
So, you see -- it isn't about the why.