The gist of it is: There are worlds and circumstances into which people want to bring kids. We mostly failed to create that world and those circumstances. If tou want to know what a solution looks like look at western democracies with high birth rates. E.g. France, Ireland, New Zealand, Denmark.
The rest is probably due to individualism. Kids cost time and energy and capitalism tells us we should live our best lives. And the best live does not involve herding 6 kids, even if they are your own.
We created this world willingly, and the low birth rate is a consequence — and so is the need for migration if we wanna stay wealthy. The thing is: now that the genie is out of the bottle we are not gonna be able to force people to have kids. We can just create circumstances in which they are willing to.
I am in my 30s and I am unsure if I will ever be able to retire. I am sure however that I will have to change my job and career multiple times throughout my life. Despite me being financially well off, I am not sure if I would be able to support kids in the future. This is not about being whiny. I just think if I had kids it is my responsibility to throw them into a world that can sustain them. This one can't.