What are you even talking about? My mother's parents were 74 and 76 when I was born and they were routinely my caregivers several times a week (while my parents were working) until I was about 6, and occasionally after that. That wouldn't work in every family, but the idea that "a 65 yr old is a dependent" is utterly unfounded as a general rule.
They're either dependent, or able-bodied, or dead. Not sure what the relative percentages are, but I doubt that "able-bodied and healthy enough to provide ongoing childcare for grandkids" is the majority here.
Looking at my area (urban Poland) there's a matter of urbanization as well - plenty of parents in the cities are first-generation city dwellers; their parents live in the countryside. The logistics (and CoL in cities, as GP noted), makes involving grandparents on regular basis prohibitively expensive.
N=1 in my case, we have one grandmother in the city, still professionally active (so limited time), and two grandparents retired and self-supporting, but living far away in the countryside (so unable to help most of the time).