I don't think you are being unreasonable OP or alone in that point of view.
I've definitely done a share of childminding, more on an ad hoc basis than tied down to regular dates. Our grandchildren come for a weekend, or part of it, probably a couple of times a month. Before ours arrived my husband had played a large part in his older grandchildren's lives, particularly when his son their father died suddenly when they were very young. Of late he has also started to take our grandson, aged 10 to the driving range having bought him a couple of golf clubs at Christmas, he made the same offer to our granddaughter but she decided it wasn't for her, so I get the short straw, or long depending on one's opinion of Starbucks to head off there with her for a Frappucino yuk looking beverage her and me a coffee then a mooch around the shops
I think I, or I should say we, have tried to pitch ourselves somewhere between being permanently available, anytime. There have been occasions when we have had other arrangements and have said so and the outright "no/never" My husband told me his mother's stock answer when he had his first children from a previous marriage was "you had them, you look after them" and although I liked her my opinion of that was "so intransigent and unhelpful, they, his parents, were fairly young grandparents then", but that attitude was pretty prevalent then. And I did say to him, "you should have said, but I looked after younger siblings after school until you came home, so actually you owe me!" he "I don't think that would have gone down well" He's always said to me "there's more to life than being a professional grandparent" but for all his protestations he kind of is unintentionally has had a great input with all of them. I suppose that the upside is greater involvement fosters a closer relationship. In retrospect I wish I'd had more conversations with my own grandparents the older I became the more remote the relationship.
Aside from my personal pov, I'm amazed at what some presume from both sides, the children who expect their parents be always on tap for child minding, no matter what their commitments, distance, state of health etc are and the ridiculous grandparents who demand time and sleep overs with grand babies when the parents aren't happy about that.