Usually these topics emerge with the same three major factors for why people aren't having children:
1. Can't afford to raise a child
2. Concerns about the future state of the world, e.g. climate change
3. A huge loss of freedom
Anecdotally, I personally feel like #3 is by far the biggest reason why people aren't having children. Every time someone in my peer group (mid 30s) announces that they are having a child, there's widespread acknowledgement that we are going to see MUCH less of them from now on.
This is a tradeoff, and those who really want children will still choose to have children. But for those who are deeply engaged in something else -- hobbies, social life, career -- or simply enjoy a low-stress life, having children would upend that. Modern living gives a lot of opportunities to go really deep into a few different facets of life, but most people have to limit that depth once they have children. It doesn't help that modern child raising seems to be endlessly shuffling children from one structured activity to another, with little help from a broader social circle.
Plus, there's less social pressure than ever to have children. If many of your peers aren't having kids, you're not the odd one out anymore.
So I think many people of childbearing age today see a much higher cost in childraising than what people saw in the past. If they aren't especially enthusiastic about raising children in particular, is it a surprise that many are choosing not to?