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

Depends on what the rwo players actually want.

If you want blood, then for sure, the winner is the last man standing. Blood for the Blood God!

If you want to LEARN and IMPROVE, which will be a better investment of your chess-playing time? Continuing a hopelessly lost game that you can already see how YOU would win if the colors were reversed? Or admitting that you lost THAT one, and playing a new game against a new opponent?

Which is likely to teach you more?

I'm not claiming that playing on a Queen down won't teach you anything. The first time. But playing on a Queen down for the twentieth or for the one-hundred-and-twentieth time isn't likely to teach you very much. Not much marginal utility there.

Grandmasters and other high-rated players tend to resign rather than play to checkmate for a variety of reasons. One reason is that they are able to recognize when the game is effectively over and they have no chance of winning. Another reason is that they may have a scheduling conflict and need to move on to another game or event. Additionally, some players may choose to resign in order to preserve energy for future games or to avoid further frustration. In general, resigning is seen as a sign of good sportsmanship as it allows the opponent to move on to the next game and saves time for both players.

For strong players, the decision on whether to play on in a lost position depends on the amount of counterplay they can get. If your opponent has two passed pawns with a king next to them and no major pieces left on the board then it's just a waste of time to play on. If you're a grandmaster and you are a piece down with no compensation in a classical game, then it's also a waste of time to play on. If you're a piece down but the position is complicated, you're strong but not at GM level, and your opponent's time is low, you might just play on and try some tricks. Such situations even offer some learning potenial, you learn to be resourceful.

Vishwanathan Anand, who later became Chess Champion of the World, once resigned a FIDE tournament game on move 6.

You might feel that move 6 is a bit early for a Grandmaster and future World Champion to be resigning, but if you check the position he was indeed dead lost.

I was going to mention that example: 

https://www.chess.com/blog/BMblogmaster/vishy-anand-loses-in-6-moves

He didn't even wait for the opponent's response. It also shows some respect towards the game and the opponent.

     Because they know when the game is hopeless, and they know their opponent is competent enough to finish them off. Also, they feel it is impolite to play on, essentially saying "you're not good enough to finish the game without blundering".

When playing against ordinary mortals, the player who makes the last blunder will lose.

With Grandmasters, whoever makes the first blunder will lose.

There are famous examples where strong players allowed the opponent to mate. One is the legendary "Game of the Century", where Donald Byrne let 13 year old Bobby Fischer finish him off:

A recent example is Magnus Carlsen allowing an en-passant checkmate against his buddy Hammer:

https://www.youtube.com/watch?v=JWdqyhWceVw

There are situations where it would have been nice if the checkmate would have been allowed to happen, as in this recent game of Duda against Giri:

https://www.youtube.com/watch?v=GhrcqNg5pbA

I also think it would have been nice if Karjakin allowed the checkmate in his final WC game against Carlsen.

It really depends on context whether allowing the opponent to play until checkmate is respectful, disrespectful or just doesn't matter. In the case of Byrne-Fischer it was certainly a sign of respect, as in the game Hammer-Carlsen game. However, when one GM enters a completely losing pawn endgame against an equally strong GM that can be converted using automatic technique, it's just wasting your opponent's time and you're implying that there is a chance they might not convert it. 

Sometimes grandmasters play on:
https://www.chessgames.com/perl/chessgame?gid=1008376

https://www.chessgames.com/perl/chessgame?gid=1066901

This is an old topic, resurrected by a spammer commenting "baller".

The reason for which low-rated players let play until checkmate is because, sometimes they are not aware that you can do it, but mostly because there is still a chance for the opponent to blunder / let draw.

Grandmasters are aware of the fact that you can resign and it is way less likely the grandmaster they are playing against does not know how to mate with three queens.

It's acceptable and quite respectful for players, more so any player from 1400 up to resign games, you are capable of seeing your position is beyond saving, it's not a case of denying you the pleasure of your checkmate over ... (Over the board you would resign a game by offering your hand🫱🫲) What is not acceptable and this does happen is knowing you have a hopless position and deciding to nolonger make any moves till games is timed out, this is very rude and bad etiquette!!?

Even though they see that the game is finished or unwinnable, they don't have hope that their opponent could make a mistake?

let's imagine you schedule a tennis game against a friend, but just before the game you have a car accident and you break an ankle and both wrists, being unable to even hold a racket

would you play the game and witness your opponent just score point after point with no opposition? now imagine your opponent is a pro player serving at 200 kph

at some point it's a waste of people's time

Have you never watched boxing? Or snooker? Or poker?

t's acceptable and quite respectful for players, more so any player from 1400 up to resign games, you are capable of seeing your position is beyond saving, it's not a case of denying you the pleasure of your checkmate over ... (Over the board you would resign a game by offering your hand🫱🫲) What is not acceptable and this does happen is knowing you have a hopless position and deciding to nolonger make any moves till games is timed out, this is very rude and bad etiquette!!?

Resigning a game is nothing new. Mic drop!