We had the same experience - raised 5 kids, not a single time did any of our 4 living parents offer to watch the kids even for a few hours - we thought that was unique to us and our parents, didn't realize it was a trend ...

On the flip side though, now that my kids are approaching child-rearing ages, both of us (spouse and I) can't wait to babysit and spend time with the grandkids when they come - we are already jealous that the in-laws might get them more than we do.