Grandmasters can play chess at a high level because they can understand the position. Understanding the position is what allows them to efficiently prune the tree. For example, if all my opponents pieces are aimed at my king-side, I'm not going to consider king-side castling, or any combination that involves king-side castling. Also, if I know that in the Yugoslav attack of the Sicilian Dragon, exchange sacrifice on c3 is normal, I don't need to think 10 moves ahead to sacrifice a rook for a knight.
Computers are unable to understand positions. Take the final setup mentioned here - http://scienceblogs.com/evolutionblog/2014/02/23/sunday-chess-problem-10/
Even a strong engine will analyze for millions of nodes, and believe that black has a definite win. Without identifying the underlying theme of the position - that if White leaves his pieces where they are, Black can never capture, there is no way to understand that this is a draw.