Why do some players have a hard time accepting defeat?

Practically every second game I play combo decks, I find myself up against people who refuse to give up even when I'm mathematically winning. I always hear about competition, playing to improve, accepting defeat, and having a positive mindset... then as soon as your opponent uses a deck you don't like, you go afk and waste both of our time. I may be a bad person, but I think it's really a question of honesty and respect that you give when you see I have all the pieces to win and there's nothing you can do to stop it. Come on, it's an online game, you don't need to act like we're at the pro tour finals.I know full well that people do it to "punish" combo players, but it's a shame that, as competition dictates, if a deck is strong, there will always be someone who plays it. So, ultimately, I don't understand why this behavior isn't a auto-ban.