I think how much babysitting/child minding grandparents want to do is entirely down to their individual circumstances and how fit/healthy they feel and what other commitments/interests they have. I can quite understand the OP point of view, I imagine if I'd had had 5 children I would feel the same.
My husband often mentions that when he first became a father early 20s, if he asked his parents to occasionally look after the children bearing in mind they were youngish grandparents, they trotted out this "you had them you look after them" Absolutely their prerogative I know, but the way he relates that, it sounds as if they were wearing their intransigence as a badge of honour. They had a large house it wouldn't have killed them very occasionally to have had the first grandchildren stay over imo. There's no way I would want to surrender my life to being a full time grandparent, life moves on from when we were doing all the looking after young children ourselves, but neither would I want to come out with a flat "no" I like to help if I am able to, A balance between dropping everything and cancelling arrangements and an outright no way! is what I aim for.