This absolutely drives me bonkers.
Both of my kids were drilled to write thank you notes immediately, as was I. DS #1 had 50 guests at his Bar Mitzvah. DS#2 wasn't so lucky; there were 100 guests and notes to write at his! They wrote those suckers for 2 months until they were done. DS #1 became his HS class secretary based on his speech that talked about how his mother drilled him to write thank you notes, thus, he was qualified to be the secretary. Ten years later, he is organizing the class reunion because of all of this "social grace" training.
Thirty years ago, when DH and I bought our first house, we invited about 20 people for a house warming. It was scheduled for 7. At 8:00, when no one had showed, we were about to leave and go to the movies. People started showing up at 8:30, and almost everyone came. This was not billed as an open house. It was a party, with a definite starting time. We almost dumped the food out and left.