That entails analyzing all of their games and finding defenses and weaknesses. But also trying to find new novelties in the openings etc. And since the opponent will do the same, you yourself have to prepare defenses for lots of potential new openings 16 moves deep or so. It's an insane amount of studying.
Of course, that pays off for future games outside the match as well. But when you know your challenger will spend half a year on this once-in-a-lifetime opportunity to beat you, you probably have to do the same. So have to say no to everything else.
