I see three big reasons why people aren't having kids:
#1: Raising kids is really hard. They're expensive. They eed constant attention when they're young, and in modern American society they need to be in a bunch of activities once they're older. And all the various tasks of day-to-day life that don't disappear: work, food prep, cleaning. I spend virtually all my waking hours on work, chores, and childcare. Being able to offload some of these (or being able to afford to offload some of these) would reduce the burden to carry.
#2: People are stressed about the state of the world. Are we going to enter an era of greater political unrest? Is AI going to ruin the economic prospects of almost everyone? Is climate change going to ruin civilization? Most people I talk to are not hopeful about what the next 40 years are going to look like.
#3: The network effect. When you're the only one in your friend group having kids, you're going to feel extremely disconnected from that group. You'll be the one sitting out while everyone goes out to have fun. But if most or all of your friends are having kids around the same time, it's more of a shared experience where you can bond over it. It's the opposite: a nudge to your childless friends to join in and have one of their own.
The thing is, none of these are really easy to solve with policy. #3 basically requires #1 and #2 to improve enough to kickstart a feedback loop. #2 is made of the big issues of our era, and won't be solved anytime soon, and certainly not for the sake of fertility. That leaves #1, where the most you can do is to give money and long maternity/paternity leaves. But it would take a lot of money/leave to really push the needle. This likely isn't politically feasible.