Because he underachieved with an extremely talented team and has shown many times that he gets out coached in big games. He and Dak are meant for each other. Big games, Dak sucks and big games McCarty gets out coached.
They win because of how talented that team is and beat bad or average teams very consistently. But when they go up against a good team, with good players, and good coaching, it's a disaster. From game planning to execution. It's been this way ever since Aaron Rodgers got him fired. And that's exactly the gripe Aaron Rodgers had on him, that his game planning was trash.
Even Packers officials say that his way of game planning was basically "Quitting on his team".
https://www.sportingnews.com/us/nfl...n-bay-playoff-record/1254aa6845bba947f5286aab
And you saw it in Dallas whenever they skirted to the playoffs surviving on how insanely talented that team is (I rarely watched em in regular season). Sean Payton, Kevin O'Connell, Sean McVay... hell maybe even Todd Bowls likely would've at least made a Super Bowl appearance with those rosters. Dak be damned.