The format of world championships (almost all classical games with very long time) mean that preparation is key to going far. Additionally in modern chess with the way points are structured, draws are increasingly common and even aimed for by players when they sense they cannot eke out the win (and they can set them up VERY far in advance at times). This means to do well in the WC you have to prepare a LOT and with the format, the games are more draining and more difficult to get through (there are a number of stories of players who "break" mentally or make critical errors in game 8+ just due to sheer mental/physical fatigue).
Just seems like the format is draining and the games aren't interesting/fun for Magnus.