Computer chess isn't instantaneous. While you can go get Stockfish and run it on your phone, the full power of the top engines only comes out when you throw hardware and time at them.

They're running the engines with a 30 minute starting clock plus five-second increment per move.

So far, Ethereal and Stockfish have each managed to win a game as White (Caruana) from the Game 12 position.

Also this site has been running live analysis using Stockfish on 20 CPUs during each game of the championship, and its evaluations frequently have changed with more time to "think":

http://analysis.sesse.net/

Computers are better but they still take a while to process the best moves, more if you're playing out a couple possible moves for each player to try to figure out the possible branches a human might go down that a computer would discard.