The answer is simple, IMO. Education, autonomy and financial health.
Women in the US weren't allowed to open their own credit card accounts until 1974. Before that, they required a spouse or father's permission/co-ownership.
Women's health is poorly understood an research pertaining specifically to women is SIGNIFICANTLY under funded.
Contraception and access to it is being challenged.
Fertility, both male and female, is being jeopardized by environmental factors.
Child rearing is not exclusively a woman's job. The choice to have children and take on the responsibility of child rearing should be shared, not assumed. Yet, women shoulder the majority of the burden, especially in circumstances where the men become absent.
Very few developed countries offer sufficient maternity AND parental leave. The US claims to be a developed country, yet it does very little for families and has the poorest infant and maternal outcomes of any developed country. Texas has the worst maternal mortality rates of the lower 48. Newborn babies should NOT be in daycare, yet it's not uncommon in the US.
School is NOT daycare.
A capitalistic government is NEVER going to put the people first. It's always all about the $$$ and keeping the pockets of the top 1% lined.
Even in mixed or social economic markets, it's a tough sell when governments are offering "incentives" for families to have babies because those incentives come from taxes they've already paid. A place like Korea, you pay high taxes to look after the super aged 20% of the population and then you pay again to have some given back if you have a baby...seems like a lot of work for little gain.
Salary disparity between men and women is 100% a thing. It's pretty significant, too.
Women perform the disproportionate majority of unpaid household work.
Baby boomers still own nearly half the real estate wealth in the US, despite representing only about 20% of the population. Affordable housing is challenging, as a result.
The world is a proverbial 💩 show. When this guy is saying he doesn't have an explanation for why people are having fewer children, it's not that hard. Socio-economic status is always going to be the most influencing factor over birth rates. People are becoming more educated and prioritizing their own self worth and autonomy and don't want to spend every last dollar treading water to have a kid.
Additionally, people are recognizing violence. For years, women used to be trapped in abusive relationships because they lacked autonomy. Marital rape wasn't outlawed in all US states until 1993!!! 43 countries still do NOT have legislation protecting women from their husbands. There are well over 100 countries that still allow child marriage. Who wants to have a kid, let alone more than one when this is the case?
Sorry for the pessimism, but I am in a decent financial position, so I could afford to have kids. However, I watched my mom struggle as she raised my brother and I in an absent father household and had to work her behind off to keep us housed, fed and clothed. If told she was going to have to ride the struggle bus for a good proportion of her life, I suspect she'd have chosen not to get on. People are better able to see that bus coming and are saying "nah, bro...not for me. I want better for myself."