Rulers vs rulers. The rulers know it, and so they're united against the citizens. 

 A cartel 

 Grok estimates that the EU's average fertility rate in 1960 was approximately 2.59 births per woman.

The EU’s average fertility rate is now 1.46

This is a reduction of 77 per cent.

Grok estimates that the public sector's spending as a share of GDP for the five major EU economies between 1960 and 2022 likely has grown by at least 50% to 100%.

By other words, in this period we have seen a shift of economic wealth from the private sector to the public sector comparable to the average reduction in the birth rates.

My best guess is that it's here we find the main explanation.

Shifting of wealth from the private to the public sector is shifting of wealth from those  produce kids to those who produce bureaucracies.

The result is production of more bureacracy and less production of kids.