Why didn't Spassky play 26. g3 in game 5 of Fischer-Spassky (1972)? Fischer's play was powerful in this Nimzo-Indian, achieving a strong knight against two bishops in a closed position. The knight became a monster on f4. But why did Spassky let the knight reach f4? The idea of playing 26. g3 to prevent the knight from getting there seems like an obvious idea. If not for f4, it's hard to find a good square for the knight. Sure, the move 26. g3 has some drawbacks, like weakening the light squares around white's king and making the dark-squared bishop even worse. But it looks like the benefit would outweigh the drawbacks. What are your thoughts?

I used the chessgames.com engine to analyze the position and was surprised to find that 26. g3 wasn't even in the top two moves. I wonder why this is? Why did neither Spassky nor the chessgames.com engine choose 26. g3 as a move?

[fen ""]
[Event "Spassky - Fischer World Championship Match"]
[Site "Reykjavik ISL"]
[Date "1972.07.20"]
[EventDate "?"]
[Round "5"]
[Result "0-1"]
[White "Boris Spassky"]
[Black "Robert James Fischer"]
[ECO "E41"]

1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. Nf3 c5 5. e3 Nc6 6. Bd3 Bxc3+ 7. bxc3 d6 8. e4 e5 9. d5 Ne7 10. Nh4 h6 11. f4 Ng6 12. Nxg6 fxg6 13. fxe5 dxe5 14. Be3 b6 15. O-O O-O 16. a4 a5 17. Rb1 Bd7 18. Rb2 Rb8 19. Rbf2 Qe7 20. Bc2 g5 21. Bd2 Qe8 22. Be1 Qg6 23. Qd3 Nh5 24. Rxf8+ Rxf8 25. Rxf8+ Kxf8 26. Bd1 Nf4 27. Qc2 Bxa4 0-1
I'd actually argue that the drawbacks of g3 are worse than Black getting a knight on f4. White permanently weakens the kingside light squares and blocks in his dark-squared bishop. Meanwhile, Black getting a knight on f4 is a bit unpleasant, but it's not necessarily a permanent problem. 

Spassky probably thought that after 26...Nf4 27.Qc2, he was holding. He missed 27...Bxa4 of course, but if he had played 27.Qb1 instead, Komodo actually gives an equal evaluation. It's funner to play Black, but finding a plan to breakthrough is nontrivial.

After 26.g3 Nf6, Black is planning to play ...Qh5 and aggressively capitalize on White's weak light squares. If White stops this with 27.Qd1, Black has 27...Bg4.