The market is a feedback loop, not a static system. Daycares may have started appearing because some mothers wanted to go back to work earlier. But, as more and more families started using them - whether to gain a financial leg up, boost the mother's career, or for personal reasons - they've become established. At the same time, the market started pricing it in - what was perhaps once a trick to boost your material status, now became the default, and these days sending kids to daycare isn't a way to boost anything - not sending them is a way to have less money. Similarly, daycare is expected in careers simply because there's more than enough mothers doing it that not doing it puts you at a competitive disadvantage.

The market rewards being above the mean, but otherwise works to make the mean cheaper. If a few % of people find a way to move above the mean, they reap great rewards - which encourages others to do the same; the mean starts to move up, and competitive pressure starts to push its value down. In the end, "the new way" becomes the default way, and initial rewards from doing it are erased.