Iirc I've read someone here in the forums complaining about his opponent resigning a game like if it was disrespectful (was something like "it's rude, I can never imagine something like this happining when playing at a table irl").
While I think that is just nonsense, fully agree with you and get mad when someone wastes my time in other games, it means that there actually are people that think that way so that might be part of the answer. Also I believe in most games it's not that easy to be 100% sure the game is over and, even when it is, probably many players are not able to fully realize it.
Moreover, I've always had the feeling that in here most people, unlike you, use the resign function when they are mad about the game (the few times it happened to me was always when I blocked them on Ticket To Ride and they resigned without saying gg) so that might be the general sentiment about the resign option and people (me included now) don't do that because they don't want to give that impression.
Last but not least, I once had a tournament game in which my opponent was out of time and clearly losing, but he was very polite and told me he didn't want to waste my time anymore so I could just kick him out because of time. I didn't want to let him have a penalty given how nice he had been so I suggested him to just resign and he said he wasn't able since it was a tournament game. Not sure if that was a rule of that specific tournament or it's applied to tournaments in general, but that might be another reason, at least in that kind of games.