//Why not?//
Because far and away the biggest problem facing mankind is over-population. The problems it will present (and already presents in many parts) will seem a walk in the park compared to climate change.
It is even a problem in the UK (where average family size is, I believe, less than two). We constantly hear of shortages of this and that. There are not shortages; there are too many people.
If the UK has small families on average why should we bother worrying? Well we worry about the UK's contribution towards "emissions" when our contributions are unarguably adjacent to four-fifths of five-eights of sod all. But still we're urged to turn off all the lights. As well as that, the people with large families seem, almost invariably, to expect the rest of us to keep them.
That's why not.