there is also the sentiment of people in their 50s that they are done with taking care of kids. they want to enjoy their freedom now. it's just an anecdote, but for example my dad tried to marry again, but was unable to find a partner willing to marry someone with kids. i don't know if that translates to taking care of grand kids, but i think it is related.

in developing countries there are no pensions for many people and the young parents provide the support the grandparents need, taking care of the grandkids is one way to ensure that this support keeps coming.