There seems to be an assumption, and I have made this same error myself when younger, that a retired person has loads of free time simply because they are retired.
In my more recent experience this is rarely the case for younger/fitter retirees. When people retire from paid work, they don't just sit around all day twiddling their thumbs, watching daytime TV or doing nothing!
All the retired people that I know are actually quite busy. Most do voluntary work of some sort; many have taken up courses of academic study or are learning a new practical skill.
Retirement is often the first opportunity many of us have to finally take up that hobby, learn that new skill, join that exercise class, give something back to the community through volunteering etc. Add to that the need for frequent visits to elderly, often housebound, parents and the practical care, shopping, maybe cleaning that needs to be done for them.
There is often very little spare time left over - and just like younger people, retired people value their spare time and look forward to the chance to relax.
I think it's a shame if grandparents don't want to have anything to do with their grandchildren - although I do think that's probably pretty rare. But I don't think that grandparents should feel obliged to make a regular commitment to childcare. Personally, I love my grandchildren dearly - BUT I find having sole charge of them for anything longer than a couple of hours is pretty tiring. I am happy to babysit from time to time but that's it. I most definitely wouldn't want to make a binding commitment to do childcare regularly!
I'm not yet retired, by the way. But this is just how I see it.