I absolutely understand what you're saying and it is really sad. Distance doesn't have to impact on a relationship, even though obviously it makes it easier to nurture a relationship when you are close by to each other. We have a similar situation. MIL/Step-FIL have never been overly enthusiastic about our dc (or us for that matter). They live 1.5 hours away, perfectly able bodied, drive, SFIL is retired, MIL has never worked more than part-time, so has 2-3 days off during the week plus weekends. We've lived where we live now for just shy of 7 years. I think they've probably visited us 5-6 times. We did used to see them for Christmases and Sunday lunch (if we drove down to them) and they would see our dd then. But MIL didn't see her more than probably 4-5 times a year. My mum lives overseas, pretty much on the other side of the world, and sees her more than that! My mum also calls and talks to her via skype and they have a lovely relationship, even though we don't live close to each other.
It's always made me a bit sad as my GP growing up lived about 20 minutes away and I saw them nearly daily. I had overnights with them. We spent all Christmases, etc. together. I was there with for school holidays, etc. I would have loved for my dc to have that same experience, but we did always know we probably wouldn't live that close. Some things transpired in the past couple years in the family which has caused a strain (it's complicated, but related to a safeguarding issue for our dc, so we've had to go NC with a family member due to risk of abuse). MIL hasn't supported us and has decided that because she doesn't support us (she supports the person who abused the child in question, not one of our dc) she would opt to no longer have any sort of relationship with our dc either because it can't be on her terms. They are her only grandchildren and likely the only one's she'll ever have (BIL/SIL aren't planning to have children at this point). She will have never even met our ds.
I know that's not quite what you are describing, but it is part of just a grandparent's general lack of interest in their GC. It's sad that she has always shown little interest in them and especially now that she was quite happy at the first opportunity to say I no longer want any sort of relationship with them, even though in theory she lives relatively close. My mum on the other hand isn't able to live close. She's an 11 hour flight away, but her relationship with my dd especially is absolutely lovely. We of course have our disagreements and she drives me mad some days. But she is a wonderful GM. The distance is unavoidable, but we make the best of it. She is also retired and she visits about 3-4 times a year for a week. She calls every couple weeks and speaks to our dd. She sends her cards and notes in the post just because. It goes to show you that distance doesn't have to be an obstacle to a good relationship.
I've cried and screamed and held on to a lot of pain because of my ILs attitude to our dc, but time has helped and I realise now that we can't mourn something that never was. It is my MILs loss that she's chosen to not be involved in her GC's lives. I hope it burns a hole in her when her friends or other family talk about their GC. I don't really feel like my kids are missing out because she doesn't have much to offer anyway. Even though there is still a part of me that wishes they could have what I had as a kid as that was pretty special.