Reasons:
$300/week per child is cheap in many cities. In New York state, caring for one infant is 76% of a minimum wage worker's income, and NY has a higher minimum wage than the US average ($18.7k vs $15.6k).
Tuition, room and board at an average private university is currently $47k per year. That's class, groceries and shelter. Not including books, health insurance, supplies, clothes, entertainment, etc. Assume they work part time to pay for the rest of the essentials. That makes surviving working through four years of college cost 12.1 years of national minimum wage pay. Big monthly student loan payments for a decade or two after college are another reason why no one can afford to have kids in their 20's or 30's.