Not to be snarky, but how did humans ever do this in the past without all of those things? I think paternity leave and maternity leave are great ideas, but they’re a bandaid.
The real problem is that society has changed to be hostile to healthy human existence. You need a car to live in most of the USA. Extended families and support are spread out across the nation. Rather than cooking together in your kitchen and playing together on a whim, everything has to be planned around commutes and— often— long distance travel. Societal expectations are that families are small. Nothing is built for large families. The list goes on and on, but over all, I think it’s a long list of things— with economics being one— that underpins this.
There are many things society and government can do to improve this situation. We aren't getting rid of cars easily. We aren't getting rid of distant suburbia easily. We aren't getting rid of employers who WANT to see people in office.
Maternity and paternity are the easiest of them all. If there was ever to be an UBI, it would be for maternity and paternity. For 2 years, the government (or employer) covers you. Go get pregnant.
Families are already struggling when only the father is working.
Work is hard for the father, and raising kids is even harder for the mother.
Now both father and mother must have a job - and that's just to barely survive.
And the government expect us to have kids?
Fucking idiotic pricks.
Unless the government start doing something to ensure that all of those shiny GDPs are trickling down, instead of ever continuously pooled up, the situation will only get worse.
The real reason is not this. The real reason is that in the past, religion meant that people saw earthly sacrifices as leading to redemption in the afterlife, which meant that having kids was a worthwhile sacrifice (as preached by the church). To this day, people that are more religious tend to have more kids, even when income-matched to non-religious groups.
The decline in religion & rise in secularization tends to map pretty well to lower fertility, and it happens to be correlated with rising wealth (which is why we confuse the two all of the time):
Big benefits for having kids certainly helps, and you can have baby booms without strong religion (i.e. under Communism), but the perceived payoff has to be there for the parents to make the necessary sacrifices.
The real problem is that society has changed to be hostile to healthy human existence. You need a car to live in most of the USA. Extended families and support are spread out across the nation. Rather than cooking together in your kitchen and playing together on a whim, everything has to be planned around commutes and— often— long distance travel. Societal expectations are that families are small. Nothing is built for large families. The list goes on and on, but over all, I think it’s a long list of things— with economics being one— that underpins this.