Classical these days is heavily reliant on technology for preparation. The world championship match gives both players a huge amount of time to research strong lines using an engine. When players finally do get out of prep, they have plenty of time on the clock to make a draw if they're not in a winning position. In shorter time controls it becomes a lot more difficult to avoid mistakes out of book. Then it becomes more about vision and calculation and less about preparation.
Often old sidelines will come back into fashion in order to avoid prep, or to lead someone into unfamiliar territory.
If you watch on a video feed, announcers will sometimes say "this position was played in city (x) in year (n) between player(1) and player(2)" this is to show that there are few games left in the database, we are entering deep prep.
Hope I helped.