I can see this from both points of view.
I do think its a class thing, more likely to be middle-class who do this. some people are working flat out, trying to survive, don't go in for much reading or writing.
I am not middle-class, was never told to send thank you notes, and I was a child in the 1960s.
I think its a nice thing to do, but shouldn't be expected. I would always say thank you in person, probably because that's what I saw my parents do and others I love, respect.
guess one is less likely to see people writing thank you notes than hearing please/ thank you' IYSWIM.
someone I know used to send clothes/ items for grandchild. when babysitting she found these items stuffed behind the kitchen sink. didn't bother so much then.
I think so many people have so much, materially, nowadays, they don't really appreciated baby gifts, because they don't really need them.
better to give a goat to a poor country family, in the name of relative. or send shoe=boxes of goodies to poor children. they really appreciate it, need it.
otherwise it can become manipulative, mere convention= waste of goods/money= disappointment/ grievance all round.