Life finds a way. Do you want me to explain deeper into how a married man and woman living together end up making babies? To be fair. Our third was an accident and it was a hard decision to go through with it. Also, with each child a momentary boost or good news was had monetarily that would soon become moot.
And if starting out wealthy is all you seem to think should drive humans having children, consider how many people have fallen into poverty or are living paycheck to paycheck that would have otherwise not had children. The economy would collapse.