Why do grandmasters and others higher rated players resign and not play till checkmate these days?

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A better question is why do average to low-rated players resign?

Grandmasters playing other grandmasters know there's a 99.999% chance they'll lose anyway if they continue to play, so resigning makes perfect sense.

Whereas with average and low-rated players in a losing position, there's a realistic chance their opponent will blunder a piece or let stalemate happen or be unable to convert a material advantage into checkmate in the endgame (e.g. king and rook is a theoretical win vs king and knight, but the player with the rook might not know how to overcome the knight's defenses before the 50-move rule kicks in or time runs out).

But it still makes sense for average and low-rated players to resign in OTB tournaments when they have more games left to play, to save their time and energy for the remaining game(s).