This has been asked now and then over the years.
My favorite response pointed out that in other sports your ability to score isn't diminished. In ball sports players aren't taken off the field. Balls aren't taken away. This sort of thing.
In chess, sometimes the pieces literally aren't able to make a threat (through any legal series of moves).
As you get better at chess, the more easily you can see when no reasonable series of moves will ever save you. Some resign too early, but for the most part it's a logical part of the game that saves a lot of time.
Why are chess players quitters? This doesn't happen on a regular basis in any other competition. Tennis players don't just quit and walk off the court if they're in a hopeless position. Football teams don't just give up and leave the stadium if they're down five goals and there's only a minute left of injury time. Olympic runners at the back of the pack don't just stop running once the leaders cross the finish line.
On top of not being a quitter even in a hopelessly lost position, I guarantee you that even at the highest levels of chess, if people who resigned had played out their games, there would be unexpected events and comebacks on some rare occassions. Here is a thought experiment. Think of all the games that have been resigned in the history of high-level chess and have the players instead play out each game to conclusion. In this scenario, can you tell me that there's not a single instance where someone who resigned would end up winning?
Chess players should stop being quitters.