Short: In the game you have the option to supplement pops with robots and eventually turn your pops into machines. It’s also a 40k mechanicus joke.
Long: Machines will supplement workforce. So we don’t need more pops when the pops are getting more efficient. Having too many spares will just add more food demand which will drain resources better allocated elsewhere. Have to balance growth with jobs and resources if you want to win. Min max FTW!
Just look at the birth rates, you can plan what you want but if people don't want children then not even an authoritarian state like China can make them.