Because of Hope, we hope to win 🏆
Another reason is we shouldn't trust most folks to convert an advantage to a win. Now if most folks are super GMs or masters, then it's probably ok to resign. But most folks are just amateurs. Add up time pressure and I think a lot lf us had seen how a losing game suddenly became a winning one because of a last second fatal mistake from the winning side. Sometimes you get stalemate and probably psychologically even more satisfying.
One more reason is if you are playing on a team, sometimes the strategy is to emotionallly and physically exhaust the winning player by making him work more so on the next match he/she is less "fresh". I have seen this employed against my emotionally explosive uncle. He was mad because his opponent didn't resign and keep nagging about it. It affected his next match and lost badly when I know he would have won that game easily if he was cool headed. Is it a dirty tactic, maybe? But it's legal. I was once a victim of a similar psychological tactic on the board when an opponent claimed a draw when it wasn't. I admit it annoyed me and kinda lost my focus and it indeed became a draw.
Because of Hope, we hope to win 🏆