Having children (and being married) is a tool for survival in harsh environments. Conversely, it can be seen as a trap/burden from a legal and financial perspective in a 'developed' country.
Interesting! It's almost as if we've evolved out the need to have children (no need for free hands working the fields, the ability to have sex for recreation instead of procreation, and children typically providing limited financial return compared to their historical role).
Edit: In the land I come from (south Italy) it was fairly common up to 50s and 60s for a couple to raise many children, as many as 7 or eight.
The reason is dead simple: being the economy largely based on agriculture and being schools not very common, more children meant more helping hands.
That need has basically disappeared.