Magnus Carlsen doesn't play against the best computers because he knows he would lose all the time?
Currently the best chess modules clearly outperform any human, be it Carlsen or any player in history. Therefore there is no point in any man-machine competition. Competitions today are either between machines or between men. What Carlsen and many others do is to use the modules to train and improve their chess. In particular Carlsen and Caruana have modified their way of playing based on the experiences gained with artificial intelligence-based modules such as Google's Alpha zero, which has allowed them to play in a much more aggressive and innovative way.