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.

For the world championship players spend the better part of a year, generally with a large team of other world class players, intensely preparing to play games against only a single player, studying their games, and finally unleashing absolutely everything they've discovered. And then there's a constant metagame knowing each game you play one day will be deeply studied by your opponent and his team, and he will come back fully prepared the next day.

Carlsen gave up his title because he no longer wanted to put the massive amounts of work in that are required to maintain it. Would he be the best match player if he started putting this work in again? Maybe? But he's chosen not to do that.