He's not a CDM!!!
He's a high-pressing fast-recovering box to box midfielder. If you play Kante at CDM he has to sit deep and can't press high - this hugely limits what he can contribute. It should be mentioned that Lampard had some success having him sweep horizontally rather than vertically, but fundamentally the turnovers he generates are of lesser value when they're closer to your goal vs. the opposition's.
If you limit Kante to sitting deep you are essentially driving a Ferrari in the slow lane. The fundamental crux of Kante's value is that he allows you to play 2 midfielders against 3 midfielders yet lose very little in terms of duels, second ball recoveries, etc. Playing him as a CDM where he's part of a midfield 3 just doesn't take full advantage of his unique skillset. He has always been best suited to playing in a midfield pair alongside a sitting midfielder who can distribute (aka a CDM) - Leicester with Drinkwater, Chelsea with Matic, Chelsea with Jorginho, etc.