I'm not going to vote, but I will share this with you and it might be your answer.
I too was left in the care of my DGM as much as possible. Both my parents worked so taking school holidays off wasn't always possible and I actually don't have a memory of them doing so.
I do remember at a youngish age, my DGM saying once that I had been a mistake and it made sense considering other factors.
Fast forward to when I had DC, I had no offer of help at all. Actually, my Mother turned up and sat on her bottom being served cups of tea by myself (multiple internal and external stitches) when they visited.
That was it. My SIL was dotted on by her MIL (her own having died), baby washed and cared for while SIL recovered. It was a chasm between our experiences raising babies/children.
When DC was growing my parents took DC out a couple of times to the movies but never for longer than a couple of hours. Then they started to arrange medical appointments during school holidays, despite me not asking at all if they would like to spend time with DC.
In the end, DD got cranky because he wanted to see a different film than what DC wanted to see, so they didn't get to see either and that was the last that they ever went out together. I could probably count on one hand how many times this happened.
So my theory is, some parents didn't enjoy or want their children when they arrived, and certainly have no intention of repeating the experience with grandchildren. This sounds terribly hurtful, and I don't mean to say it to upset you, but perhaps this goes some way to explain grandparents behaviour... ?