Because if the fertility rate stays the same there will become a point where societies will become too small and disconnected to maintain technology. At that point people will probably have to fall back on farming in order to survive. When that happens you’ll have to maintain large families in order to keep everyone fed.
It’s not like our birth rates are falling due to some outside cause like disease. It’s because modern societies don’t require many children in each family. Give that reason to have large families back and the birth rates will explode.