Exactly! Socialiation and above all decency.
We used to live on a different continent from both my H's ILs and my mother. I used to sit with my kids encouraging them to write thank you letters and drawings to my mother when she sent presents, and out of politeness did the same for presents from H's relatives. My H's relatives would praise H to the sky about how polite and well-mannered his kids are, while complaining about the other grandkids, how they never even heard whether the presents arrived, let alone whether the kids enjoyed them.
My H gloried and revelled in all the praise.
Then my mother died, and I realised that I had unwittingly become H's social secretary for his dealings with his own family. I was still sitting with the kids, helping them write thank you letters etc to IL presents, but I was starting to think, "Wait. H should be doing this, it's HIS family."
Around this time, a conflict arose between me and SIL, and rather than let us work it out together, MIL waded in to defend her daughter and attack me. Which let me know in no uncertain terms that I had only been tolerated as an addition to MIL's family, rather than being loved and appreciated and welcomed.
So I withdrew and told my H that he should do all the thank you letters, AND send them photos of the kids (these were the days we had to get photos printed and send them by snail mail) - I had been doing all that too.
Did he do any of it? No. Cba. His mother complained, his sister complained. And guess who got the blame? ME! Now I was even more evil than before, because I was not continuing my social secretary work for H. AND unbelievably, my H also had a go at me, saying that I was being Old Testament and unforgiving and I was harming the relationship between the kids and their GPs! TF!!
You can be sure he got thoroughly scorched for that. But this entire experience showed me how patriarchy and socialisation creeps up on you, and how before you know it you're expending tons of energy and time on your H's personal relationships with others. It's rubbish and women should stop doing it. Men need to learn to work harder at their relationships themselves, not expect women to do all their emotional (and other) labor.
Later I found myself still getting H to get the kids to write thank you letters to H's nice relatives, who I liked and were frail and elderly, because I knew how much it meant to them to get the letters from the kids. Just like in OP's situation. But at least my H DID do it, and didn't tell me I was nagging, because actually he loved those relatives too. And I made it clear that I expected them all to do this by themselves eventually. And in fact, they all - including H - learned and now he and the kids have their own relationships with them, at least the ones who are still alive.