It feels like the cohort GM player pool is mentally cooked against Magnus.
Youngsters like Lazavik during the Speed Chess Championship or Sindarov in Freestyle were the most recent convincing wins against Magnus, but the historical mental edge that Magnus comes into each game after beating the brakes out of everyone is hard to overcome.
Magnus' time will come! But not today.
Are you referring to the odd individual game? Magnus beat Lazavik pretty badly in the SCC and knocked Sindarov out of the Freestyle final.
I don't think it's merely mental albeit it seems like even nervous Carlsen is cooler than his very focused opponents (see game 3 vs Fabiano where caruana had a completely winning position after carlsens blunder).
Carlsen has spent the core of his career mastering two aspects historically underlooked aspects of the game.
The first is the endgame, and there isn't much to say there. He's by far the best end game player by far and it's not even close.
The second are drawish locked positions where most GMs can't but see a draw. Carlsen realized that in order for it to be a draw his opponents still have to play perfect and he focused a lot on accumulating small but convincing advantages in those kind of games.
Another thing that should not be overlook: mental strength, like you point out.