Here's an excerpt from Yasser Seirawan's book on the match, No Regrets:

Right after the game Bobby and Boris held a postmortem and considered that the position after 17.Nxh6 was critical. Boris was sure that 17...f6 was a mistake. The players immersed themselves in the forcing sequence 17...Bxa1 (probably the best practical try) 18.Qxa1 Qxd6 19.Qxh8+ Ke7.

The next day Bobby, Eugene Torre, Svetozar Gligoric, Yvette Nagel and I spent a late afternoon analyzing this position. It is an excellent position for practical work. I suggest you take a few minutes and look at the lines following 20.Qxh7 and 20.Qg7.

Initially, Bobby was strongly for 20.Qxh7!, munching a pawn. He got bogged down over the line 20...Rf8 21.h4 (to clear the back rank and pound home h4-h5) 21...Qd2 22.Re3. White seems to be on a joyful attacking crunch, but his pieces are misplaced: 22...Qxc2! 23.Qg7 (since 23.h5 runs into ...Qd1+ and Qxh5+) 23...Qc1+ 24.Kh2 Qc5!, again restraining h4-h5. Now Black has two passers on the queenside and his king can trot to safety.

Fischer spent a lot of time trying to make 25.e5 Bd5! 26.h5 work, but came away dissatisfied. At length he was talked into declining the h7-cutie. "Man, I really want that guy!" he exclaimed. We began looking at 20.e5 Rxh8 (20...Qd2!?) 21.exd6+ Kf6 22.Re7 Bd5 before Fischer's "Nah!" ended things there. Finally, 20.Qg7 Rf8 21.Ng8+ Rxg8 22.Qxg8 a5!? (White's queen is trapped) 23.Qg7 a4, when despite being an Exchange down Black is still kicking. Indeed, the whole line isn't forced, as Black doesn't have to sac the Exchange. Bobby was vexed. "You guys are busted. Give me a sec to find the killer!" Finally Bobby said, "First, give me my pawn!" and produced 20.Qxh7! Rf8 21.Qg7 Qd2.

You must have been very, very, very, disappointed.

<he never defended his title against a young karpov>

Don’t worry Lennonfan, going into the last game of
of the Baguio City World Championship it was tied 5 to 5. (with 21 draws)
(such win rates had never been seen in modern chess)

By 1978 Karpov was already showing the world there’s a new sheriff in town.

If he was 5-5 (with 21 draws) with Korchnoi, its unimaginable how badly he would have beaten young Fischer.
Fischer had to win his matches 6-0, 6-0, just so he could rest by the seventh game.

For record Karpov also decimated and destroyed Korchnoi in the 1974 match by the unheard of:

The Houdini evaluation after 7. b4 is -0.07, that means White has given up a bit(0.05) of advantage but is still fine, it's an equal position...