It’s not anything your or your wife have failed to do. This is very common and is the norm I think, for people of those ages, 17, 25, 31, to be focused on themselves, their friends, their romantic relationships, their education, and work, buying a house etc. It is a very busy time for them as you’ve told us, one has just finished a PhD, one is in midst of school, the 31 year old probably immersed in building a career and possibly long term relationship.
So, I don’t think they’re being selfish, or intentionally thoughtless. It is part of becoming independent and building lives of their own independent of parents, grandparents, siblings.
There is often a gap of time where we children, then later our own adult kids “disappear” from our parents’ lives, our lives, as they do these things. Late teens through early 30s. Then they reappear, realizing they want and need closeness with the family they grew up with, the people who knew them as babies and children.
That said, yes, they should be sending cards, thank yous, and visiting you and your wife. They are also of an age, maturity, and physical fitness that they should be helping you out a bit, absolutely. Their parents probably try to encourage them to do these things, and they’re oblivious. Remember, they don’t know what it is like to be older, less mobile, needing help, wanting closeness with family. Yet.
Perhaps a talk with your children about your feelings and together you can figure out how to encourage the grandchildren to be more involved with family and their grandparents. 💐