He's played next to Pogba though. Pogba clearly has that role with the ball, as he does for France. Fred and McTominay gets used because Fred performs better in it and Pogba - Fred isnt as successful as Pogba - Kante is for France because Fred is half the player Kante is.
Again, Casemiro is a better ball winner and better on the ball than Fred is. Fred can play next to Casemiro because Casemiro does everything, Pogba can play next to Kante because Kante does everything. But when we've played Fred and Pogba defensively we are poor because we dont have a Casemiro or a Kante in there.
There's a few things here I want to address.
1) Even when Fred plays with Pogba he quite often is the one acting more as the deep playmaker while Pogba pushes into more attacking areas. I don't remember it being that way in their earlier matches, but it does seem to be going that way more and more as time passes (it was very obvious in one of their most recent matches together).
2) Pogba-Kante isn't the France midfield. It never has been. It's always Pogba-Kante and a third central midfielder, sometimes named in the midfield and sometimes nominally named on the wing but obviously helping out just as much in the midfield.
Since the start of 2018 Pogba has started 28 times for France. In that time he's played in a two man midfield four times, against the heavyweights of Iceland, Andora, Kazakhstan and Bosnia. And against Iceland (the highest ranked of those four teams at 52nd in the world) France was losing 2-0 with that formation until they bought on a third midfielder and later took Pogba off, after which they got it back to 2-2. Every other single time he's started for France he's had two of Kante, Matuidi, Tolisso, Rabiot, N'zonzi, Ndombele or Sissoko in there with him.
Interestingly, France are much more likely to only play two actual midfielders when Pogba isn't playing. In the 16 games that he didn't start in during that time there were 9 times that they only used two midfielders. That says a lot IMO.
So if even the great Kante, who is double the player of Fred according to you, can't be trusted to cover Pogba by himself in the international arena (where almost all opposition is comfortably weaker than most of the teams we face in the league), why are we expecting Fred to? Or anybody else for that matter? People talk about Rice or Ndidi allowing it to happen but I wouldn't be holding my breath. Same with Casemiro. That's an issue with Pogba, not Fred.
3) Kante and Casemiro are better than Fred. Not many will deny that. Fred's not 'half the player' that they are but he is below them. But then neither of them play as the main midfield playmaker of their team like we expect Fred to do most of the time, which was my point in the post you replied to. How good would Kante or Casemiro look if they were played next to McTominay and asked to be the main one controlling the team around? Casemiro obviously has Modric and Kroos, while Kante's best form has happened next to Matic (back when he was good) or Jorginho. Also Drinkwater back in their Leicester days, but that was obviously a very different playstyle where their midfield didn't expect or even try to control the game.
We should be trying to use Fred in exactly the same way that Kante has been most effective for Chelsea IMO. Next to a defensively solid playmaker who can be the main 'controller' of the team while also doing plenty defensively so Fred/Kante can press and harry all over the field.