Because children are expensive, in time, fiat, and opportunity cost while the freedom of being childfree is free if you choose to have less or no children. Social media allows for the socialization of these ideas at speed versus historical transmission rate of culture and social norms. This is covered most robustly in my Birthgap video citation above, additional citation below (by the same author as the tweet that is this post).

Terra Incognita: The Economics of a Shrinking World [pdf] - https://news.ycombinator.com/item?id=49352811 - August 2026

> "As of 2026, humanity is likely below replacement fertility. That has never happened before, not in wars or pandemics. But the real surprise is that the fall has been concentrated in low- and middle-income countries and among poorer and less educated women. We fit a single-factor model to 236 countries since 1950: the common component peaked in 1978, and what drives fertility down today are country-specific trends, 219 of them negative and not one leveling off. None of the commonly cited mechanisms can account for this pattern, so we offer a conjecture: modernity itself, which makes a third child expensive and childlessness cheap. Children come in integers, so it takes very little to move a cohort’s fertility rate from 1.8 to 1.3. And nothing in an economy pushes fertility back to 2.1. We close with the main economic consequences, in particular slow growth."

> There has to be a backstop somewhere otherwise you get infinite regress.

In summary, the global population boom happened because women were not educated and empowered to assert their reproductive choices. They are now. We are currently performing neutral fertility rate discovery via who does and does not choose to have children when it is a choice, versus when it is not a choice or forgone conclusion. A byproduct of this was economic growth, which is coming to an end, and will not repeat in our lifetimes based on demographic trends and trajectories.