I think what bothers the op is the lack of mentchlichkeit hakaros hatov is a very important midda and yes every bar mitzvah bochur dreads sitting down every night and scribbling out these notes but they are now considered halachically adults and when your an adult often you have to do things that are not pleasant but you do them anyway because you have a responsibility to do so, so in essence the thank you notes are their first glimpse into life as an adult.
As far as wedding/baby gifts I just don’t know.