You'd be surprised!
In times past - within ALL sports, games and competitive activities - there was greater emphasis on observing unwritten codes of conduct. In the present day, there is less awareness of - and less acceptance of - such protocols. The focus is on conforming to the minimum requirements of the written laws.
There is no rule that your opponent must resign in this case. He did not.
I was playing a game 20min/game on live chess. After move 20. I managed to get a good position. He had just under 14 min left and I had about 15 min. He stopped making moves. I waited I figured that he was trying to figure out what to do. Then when he had less than 5 min. left I knew that he has just abanded the game, but there was a chance that he was just waiting for me to go away and he would make a move and win on time. So I waited.
Does this behavior happen often? It is not so difficult to click the resign button is it?