Because they are garbage players who hope you will make a mistake and click "yes", and they will earn a draw even though they got completely outplayed.
It's the same with the people who shuffle their lone king back and forth, hoping you will disconnect / make a mistake so that they can earn a win on disconnect or a draw by stalemate.
This usually happens more so if they make a bad move and I instantly punish it. I just calmly decline it and most of them will resign. Some will waste all their time but it's not my problem that you made a mistake.