It is because they CAN. They never wanted to reproduce in the first place. And the reason isn't even the democracy or "rotten Western values" - they die off even faster in authoritarian, patriarchal Eastern countries, free and unfree alike. It's simply economic growth.

Give me any way of "making people reproduce again" which isn't overtly dystopian-totalitarian and i will accept that promoting "as much immigration as possible, not letting in only known criminals" was a bad idea.

Sure government can just start having babies for itself. That will be real cattle herding.