Best to contextualize this with the knowledge that the initial transition towards pastoralism and agriculture (ie the Neolithic) brought a major negative impact to overall human skeletal health.

There are hundreds of studies examining this question and generally coming to the same consensus, but here’s a couple more narrative and less jargony ones.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4466732/

https://pubmed.ncbi.nlm.nih.gov/21507735/

The overall takeaway is that agriculture resulted in a severe downturn for human health, but renders out to a net contribution to fitness because of massive increases in ovulation. Women even in extant hunter-gatherer populations experience a total number of ovulatory cycles somewhere around an order of magnetite less than women in agriculture societies, even in what we would today call “developing” conditions, or in older research is sometimes called “third world” or “unindustrialized”.

https://www.jstor.org/stable/4602928

https://www.jstor.org/stable/4602772

https://www.pnas.org/doi/10.1073/pnas.1524031113

From the third citation:

> We examine the effects of sedentarization and cultivation on disease load, mortality, and fertility among Agta foragers. We report increased disease and mortality rates associated with sedentarization alongside an even larger increase in fertility associated with both participation in cultivation and sedentarization. Thus, mothers who transition to agriculture have higher reproductive fitness. We provide the first empirical evidence, to our knowledge, of an adaptive mechanism behind the expansion of agriculture, explaining how we can reconcile the Neolithic increase in morbidity and mortality with the observed demographic expansion.

There is no precise physiological answer as to why, so if someone says they know, they’re either lying or not well-read enough. Agriculture means lots of carbohydrates, and insulin has major effects on sex steroid balancing via its modulation of aromatase, so that is probably a contributor. The massive decrease in population mobility also means a less energy-stressed environment, which is a potent suppressor of ovulation, and also of lactation. Lactation itself also suppresses ovulation, and breastfeeding duration is markedly longer in extant non-agricultural populations, though these groups tend to be somewhat hybridized and not nearly as mobile as their (and our) Paleolithic ancestors, so they should not be taken as a perfect model for those extinct groups.