Taking the recent Anand-Carlsen match as an example, I know that grandmasters spend incredible amounts of time in opening preparation, and Anand's play to refute certain lines of the Berlin defense was impressive.
But once reaching game 10, 11, or 12 and desperately needing a win, why don't players make an effort to force the game off into some opening line that the opponent almost certainly hasn't prepared for? Not into some fringe or unorthodox opening, but into solid openings that are just much less familiar for the opponent.
Much the same way that an American football team must spend at least some small amount of time practicing "trick" plays, like on-side kicks or throw-back passes, or much like a football (soccer) team might practice very special free kick formation for late in a game when a goal is needed very quickly -- shouldn't it be normal to prepare for a championship match by spending at least a small amount of time studying some very different openings that the opponent has not faced recently or does not appear to be as strong in?
I'm not endorsing these as candidates for specifically Anand v. Carlsen, but I might have expected to see some different variations of Sicilian, or to see Dutch Defense, Benoni Defense, or to see Reti opening, and to play in lines that don't transpose easily to positions that are similar to the main lines played (Grünfeld, Berlin, etc.).
I'm not saying Anand's openings were the problem in the match -- he often played well with great opening prep and blundered later on.
I am just asking why it is so uncommon to see players deviate to some "special preparation" in late stages of a match when they are trailing and need a win.
Wouldn't that be one of the scenarios that they prepare for specifically prior to the match starting? "What if I find myself down 1 point late in the match and my opponent has shown clear strength in the most frequent openings from the match? How will I steer the game into unknown territory?"