The economics almost certainly play a role, but I think the better way to think about it is how we economize time too.

if you are chasing a career, putting in 40,50,60 hours a week - how can you take time off to have a kid? who is going to take care of the kid?

Increasingly having kids has gotten more expensive - housing, childcare costs, and general expected investment/supervision of children. In agricultural societies, kids often helped out with the farming; send them to school and they are around less to help. Say that kids can't roam around outdoors unsupervised, and caregivers have to spend even more time watching (older) children. Etc. And as people increasingly move further from where they grew up to chase good jobs, that means they are on average further from their families who would have helped with childcare in previous generations.

The economic realities factor in too - people are waiting longer to get married because they want to date financially stable people, and financial stability is on average taking longer to achieve. But if you had to move to a more expensive city, further from family... that's a recipe for couples where both work and perhaps have to work to make their finances work. Babies have become a luxury item in these higher cost of living places.

if we want more children, we need to make it easier to be a parent. Cheaper / free childcare, better parental leave policies, and cheaper cost of living so that people who want to be stay at home parents can have that option.