All you can do, Diana, is make your own decisions with your own sense of personal integrity.
Who knows why people choose what they choose? Perhaps Cabo in January would be a relief for her if she lives someplace cold. I can tell you, I HATE being too cold. It physically hurts for me and I'm only in my 40s. So I can see why that would be an attractive option. (although I wouldn't choose Cabo)
She may be the kind of grandma who just really isn't that into little kids, more wanting to do what she wants. One of my sisters has an MIL like that-- when the in-laws visit or want to go on a trip, it's very adult-oriented. That's hard on the parents and not as satisfying for anyone. And for some people of any age, a gaggle of kids can feel overwhelming.
Or if your relationship has you both a bit edgy around each other, she may just be practicing avoidance.
I don't know. My guess is that there is no 'average' grandma: in reality, what you are wishing for--and understandably so-- is the kind of grandma you WISH your kids could have: one who loves and appreciates their grandchildren, who is warm and safe and emotionally healthy. There's nothing wrong with wanting that. My mom is bat-shite nuts and not in our lives-- believe me, I am right there with you on wanting a functional and loving grandparent for my son.
Boy, there are times we really just WISH they could do better and be the person we'd love to have in our lives and in our childrens lives. And the reality is that sometimes, some of us just don't get that.
For what it's worth, I've found that making sure to create our own healthy 'family of choice' with beloved friends has provided Kiddo with quite a few wonderful, healthy women who have adopted him into their hearts and families. I think that's the best we can sometimes do.
Sending you a hug, because I'm right there with you. :)