In the Seventh Seal why does Death let the chess game happen?

Ingmar Bergman was partially inspired to make The Seventh Seal by a painting by medieval Swedish artist Albertus Pictor which depicts the personification of death playing chess with a knight. As a result Bergman chooses to portray Death as being an enthusiastic player of chess, willing to extend Antonius Block's life seemingly for no other reason than because he enjoys the challenge.

KNIGHT 
    You play chess, don't you?

A gleam of interest kindles in DEATH'S eyes. 

  DEATH 
    How did you know that?

  KNIGHT 
    I have seen it in paintings and heard it sung 
    in ballads.

  DEATH 
    Yes, in fact I'm quite a good chess player. 

  KNIGHT 
    But you can't be better than I am.

I assume you ask "why Death allowed Knight to delay his death if he'd told him:

there's no delaying this"

To which my answer is: That's not what Death said.
First, Knight knew that Death had been following him.

Death: I have been for a long time at your side.
Block: This I know.

Second:

Block: Wait a moment.
Death: You all say that. But I give no respite.

Death doesn't give respite. Now for the magic of the movie: Time is non-existent in the movie. Look at how it begins:

there was silence in heaven about the space of half an hour

And just before Death appears, there is silence. Pay attention to how long it lasted (hint: it's about background sounds). Was it for 30 minutes?

Third, Death is seen "personally" being responsible for the death of a person, so we know it didn't have to wait for a person to "die of something".

And fourth: Knight was dead from the beginning. The whole movie is a Dance Macabre, which we are only able to see through the eyes of Jof at the end.

Playing games with Death has a long history; chess is common, but many other games show up as well. So we would have to look much further back to determine why....

Seventh Seal may be the most famous film use, but such gaming also appears in Bill and Ted's Bogus Journey:

(where Death loses, then demands best of 3, best of 5,...); in The Dove (also Bergman, with the heroine playing badminton); and in the comic xkcd with Dungeons and Dragons.

A large list of other instances in film, literature, etc., is available at [https://tvtropes.org/pmwiki/pmwiki.php/Main/ChessWithDeath]

Death seems to have this grim sense of humour throughout the movie, and isn't he kind of known to be willing to prove this point - that you can't escape him by chess or otherwise, you'll always lose.