Because eduction and prosperity reduce the brith rate. You don't need that many children to care for you when you're old, if you can expect to get healthcare. You don't shouldn't get accidental children if you're educated about sex and protection. In an educated society you will have fewer children later in your life, because perhaps after school you will also go to university, delaying the moment where you feel stable and settled enough to start a family. The more educated you are, the higher the chance that having children is a deliberate, careful choice, and not just something that happens. And last but not least, politics play a huge role in what incentives you have. Especially for this point, I recommend the Kurzgesagt videos about South Korea and Germany.

• https://www.youtube.com/watch?v=Ufmu1WD2TSk
• https://www.youtube.com/watch?v=n-gYFcVx-8Y