Our ancestors had children out of necessity.
Infant mortality rates were higher, life spans were shorter. Looking at pre-industrial revolution England.
- The infant mortality rate was 140 out of 1000 live births
- Of those that survived infancy, 30% died before the age of 15 as a result of disease. This percentage would increase in urban areas as diseases proliferated in densely populated areas. During the Great Plague of London in 1665, 80,000 - 100,000 people perished and it is estimated that over half of the deaths were children under the age of 15.
- Life was harsh. Young boys were doing hard manual labour and working dangerous jobs at young ages, frequently resulting in disabilities and fatal injuries.
- Poor rural families needed sons and daughters to work as no help could be afforded. Dad couldn't pay farmhands, he was going to have sons.
- Wars were frequent and lords needed men to fill their ranks. If your lord pressed your 17-year old son in to service then you better hope you had more sons to help when the harvest came.
- Life expectancy was way lower than what it is now, especially among the poor. If you made it in to your 40s you were lucky.
If you go back to the Middle Ages life expectancy dropped in to the 30s. A quarter of all births were dying in infancy. As much as 40% of births would not make it to adulthood.
That is why are ancestors were having as much as 8 kids, because depending on time period less than half would make it to adulthood. At least one of them was dying in infancy. At least another one or two would die before the age of 15 and you are probably losing another one before their 19th birthday for whatever reason (war, disease, fatal injury while working). Then of course your father is probably going to be lucky to see his 40th birthday, so you better hope you have one or two sons that can take over if he falls ill or dies.
In the 21st century, no need to be pumping out kids at the same rate our ancestors did.