What you haven’t factored in, is how people feel physically as they age. You’re looking at it from the perspective of someone who is younger. I know that young parents mostly work full time and juggle that and child care and I was someone who was determined to help out with future grandchildren, when I was a young mother and always busy.
When people are retired, especially these days, they’re generally less agile, less energetic and tire more quickly, even those who are in reasonable health.
Theres no way I could look after young children now, but my grandchildren are older, so I don’t need to. When they were small I was still working full time, but babysat, did overnight and weekends. Later, I collected from school, did more weekends and holidays. I absolutely couldn’t do it now.
It creeps up on you, this aging thing and you just can’t whizz round like you used to. But you won’t know that when you’re young, because it hasn’t happened. Maybe retired people want to just potter, visit places, even laze around, whilst they still are able to get out and about. It isn’t often that they don’t care deeply about the grandchildren. They just can’t commit to a regular care role.