[weedfroglozenge]

Because they are ending the game before it gets to that stage. Similar to seeing check mates 3, 4, 5 moves in advance, they can see with perfect play that the reduction in pieces will end in an end game with insufficient material.

[beardyw]

Exactly. I find it hard to see it myself, but I often play out games like that with an engine and very often the pieces just peter out to a draw.