It’s like the difference between sparring with a teacher vs sparring with Tyson Fury. You don’t learn that much from being obliterated by Stockfish. Stockfish is too strong.

An engine which is tuned to beat Carlsen would demonstrate his specific weaknesses, for example specific issues with an opening he plays.

For his opponents, they already try to do this. “He plays this opening a lot” is a model of Carlsen, just limited to the first few moves of the game.