Actually we don't "all know that Fischer was the greatest player of all times." Try speaking for yourself occasionally. Karpov and Kasparov had much longer careers, higher peak ratings and neither one seemed to have had a debilitating mental illness that prevented them from playing after they became the world champion.
I believe the OP said Fischer was the greatest player of all time and I tend to agree. We weren't talking about length of career or peak rating (don't forget ratings from one era to the next don't lend themselves to precise comparisons).
If you consider Fischer's rise, the environment where he grew up (USA vs Eastern Bloc), the quality of his games and results vs legendary players of that era - Earth has never seen a chess player quite like Fischer.
Never fails though whenever Fischer's name pops up in the forums that the anti-Fischer trolls are not far behind. Of course Fischer was screwed up in the head. Anybody consider the possibilities that Fischer's mental illness was part of what helped him get so good at chess?
You've not heard of Pillsbury? He was better than Fischer.
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Fischer would refuse to play ;)