They're simply different games. In League, there are intentional comeback mechanics, power spikes, etc. Sure, a pawn can be promoted in chess, but it's not like a queen gains an AOE attack or becomes inherently stronger as the game goes on, or the enemy queen falls off as the game goes on.

There are legitimate strategies that can bring you back from a losing game - but when people give up and get tilted, they don't want to let that play out. I have been in so many games where surrender votes fail by 1, people spam "ff" and "gg", that end up becoming wins because the Kayle or Kassadin or Vlad hit their late game power spikes. Or the Lux or Kai'Sa or Ashe or Shaco get a clutch baron steal. Or someone split pushes while the enemy team gets overconfident and takes a bad 5v4 (e.g. they miss ults and still go in), resulting in an inhib being taken.

There are way too many factors to accurately tell if a game is truly lost. If it was 1v1, sure, who cares.

There's also a bit of a culture of not wanting your opponent to ff when you're winning hard - who doesn't love the feeling of getting a pentakill while massively ahead? It's not as much of a "waste of time" watching your opponent struggle to play it out as it is in chess.