Because that's how Solskjaer likes to play? He uses McFred as a combination, since they both together make up one DM - Fred with his interceptions and McTominey with his tackling. And both with their energy.
You need a proper DM style player to play Pogba through the middle since the majority of the defensive onus has to be done through them. Otherwise, things like this season will happen - playing Pogba through the left and Rashford through the right - RAshford is ineffective on the right and Pogba on the left means we are completely short of creativity in the middle.
Even back from Fergie, his midfield used to be Keane and Scholes - a Box to Box and a very deep lying playmaker - didn't need a DM so much when one of your midfielders is very deep and can ping passes like nobody's business. Then, it was a combination of Scholes and someone else - Giggs, Carrick, Park etc - all of which involved atleast one player who was deep and could properly pass, negating the most obvious need for a DM. Even later on, Pogba was paired with Matic, who dropped deeper and played out passes.
Now, we have McFred, who can't pass properly and has no creativity. Pogba always like to roam upfront with the ball (beat the press and carry the ball) through midfield, so he is not suited as someone who stays back. So, the need for a proper DM style player. Think similar to France, where the defensive duties fall mainly on Kante, freeing up Pogba to roam forward, link up and lay on passes.
Additionally, I wouldn't be looking forward to a 2-man midfield of VdB and Pogba - unless VdB is staying deep. Both of them like to roam with and without the ball, which affects shape.
