they met in Japan, but didn't agree to play
Why didn't Fischer play Karpov

I agree with Darth_Algar's take on the reasons why the match didn't happen. Also Fischer's ideas of goals in life changed when he visited the Phillipines in 1973. Call it a new era. He didn't have to be a one man army in chess anymore with a goal of reaching the top. There were other things to do in life.
i think he achieved everything he ever wanted so i dont think who ever it was he was going to retire world champion playing spassky almost killed him MENTALY

Kasparov made the mistake of boring the world to tears moping around with the Berlin defense against Kramnik. It was difficult to take him seriously after that. At least Fischer quit while he was ahead.

I don't see how these were unreasonable, since they were to be applied equally to both players. Karpov did play unlimited number of games in the first match against Kasparov. The last was in the tradition where the champion should be clearly out-played in order to be declared defeated.
"Challenger had to win by two games." But you can't see how this is unreasonable, seeing as how Fischer was not the one doing the challenging. LOL

“In his book Chess Duels Yasser Seirawan has drawn attention to the fact that the legend of the apparently lone fighter Fischer is a myth. In reality Fischer had all the relevant people from the US federation behind him for support. Leroy Dubeck, president of the USCF from 1969 till 1972, had agreed with the executive director Edmund Edmondson that all the resources of the federation should be gathered for the project ‘Fischer plays for the World Championship’. For this purpose they also used all membership subscriptions of the USCF. In addition Fischer had the support of Fred Cramer, also from the federation, as his legal adviser. Amongst the chess players on whose help Fischer could count were the grandmasters Bill Lombardy, Lubomir Kavalek as well as Fischer’s close friend the international master Anthony Saidy. A special role was played by Lina Grumette, who was a sort of mother substitute for Fischer and whenever necessary offered him refuge in her house.”
In addition to many people at home, Bobby had considerable help from the president of the FIDE, during the championship match itself. Fact is, according to the agreed upon match rules, Fischer ought to have been forfeited, but wasn't. Even Spassky managed to help Fischer, by accepting these "accommodations" to the challenger's many "demands."

I remember reading an article back in 1975 that Charles Kalme wrote who was a very good chess player and a mathematician. He proved that the challenger in fact had a better chance than the old 24 game fixed match.
Yes the FIDE was "controlled" by Russia back then and still is.
Karpov when he went to defend his title that was won without and games had everything Fischer ever wanted, they had no problem giving it to him.
Fischer would have destroyed Karpov in my opinion at the time. he was that much better than anyone else and probably would have held the title for about 8 years, Kasparov would have broken through and beat him when Fischer got older but no one before him.

Fischer was the Rambo of chess, or was it the Clint Eastwood ? or maybe Roy Batty, the light that burns twice as brightly only burns half as long, clearly though if he kept playing tennis and riding horses he could have beaten Karpov and Kasparov 6-0 too.

Rambo, agreed; and carried his Rambo serrated survival knife while in the PI with the little sewing kit and matches in the hollow handle. Imagine the ratings if he, Karpov and Kasparov were on the Naked and Afraid show.
And ping pong - he was a genius at that also.

The bottom line is that we will never be sure, since we can't travel back to 1975 and get inside Fischer's head.
All the arguments for Fischer being afraid of Karpov or being a weaker player than Karpov can be dismantled just as easily as they are constructed.
People who point to Bobby's 3 year layoff overlook the fact that he went into semi-retirement twice during the 60's, only to come back stronger each time.
Those who still cling to the idea that Karpov was stronger than Fischer in 1975, should be reminded that Karpov himself thought his chances would have been around 40%. Spassky who faced both men only 2 years apart, thought Fischer would have won in 75 but Karpov would have qualified again in 78 and defeated Fischer. Add to that the fact that Karpov never achieved Fischer's peak rating, in spite of a longer career, better training, more advanced chess theory, etc.
Karpov put it better than anybody else could, when he stated 'I don't want to claim that Fischer was afraid of me. I think Fischer was afraid of himself'.

I agree with cashcow8's explanation. I remember 1972 as my parents had taken me to a chess meeting for Game 6. I was a tad too young to grasp Fischer playing white queen's gambit declined.
I think Fischer was worried that he would be out prepared by a Soviet team and like every champion eventually beaten by a challenger.
Alekhine was the only champion not to be beaten by a challenger.

I tend to agree with stuzzicadenti. In my opinion, there is not such a thing as "the best of all times" in any kind of human activity.
Fischer was the best for some time, than Karpov, followed by Kasparov. Today Carsel (??) is the #1. In the past, Petrosian, Alekhine, Capablanca, and others had their reign.
Also, I think it is not possible or fair to compare one of these great chessplayers with anyother that live in a different span of time. Life today is, eg, completely different from the Alekhine´s time. Resorurces are others, competitive chessplayers also.
There is a "best" chessplayer for each moment in time.
Saludos.
fischer could have played karpov even after the 1970s
but he never tried
maybe even play a game of checkers after the ussr ended?