I think social safety nets are an under-reported cause of this. 80 years ago, children were your retirement plan. Long term stable value pensions and 401ks, along with government ensured healthcare and social security, weren't a thing. If you got to 70 without kids, you would eventually not be able to work and probably die poor.
Now, almost everyone thinks not having kids could be an option, because of course they'll have some savings (the rest of society's labor), and government healthcare, and the government won't let them become destitute.