That "some other guy" was the undefeated world champion for 15 years, the world #1 player for 21 years, and more. He was much more dominant than even Magnus. Yet it was a similar story for him where his match dominance ended well before his tournament dominance did. Even after losing the world championship title, he would remain world #1 for about 6 more years.

Maintaining dominance against a peer (and their team) who spend the better part of a year doing nothing but preparing for you, is very different than maintaining it in the relatively far more casual environment of tournaments. And there's a big ego trap here. Quitting is easy, especially when you're on top. Continuing to fight until the day you fall is hard, and requires immense character.