Pogba is an attacking #8 or a #8/10 hybrid. Players who play the position he plays(but not necessarily have the same style or ability) are players like Iniesta, Grealish, Fabregas, Ballack, Havertz, KdB, Eriksen, Maddison, Cazorla etc(even if you dont agree with the example, the point still stands). These are players that when you play with a #6, and two #8s, are the furthest forward among the midfield 3.
These players are not known for their defensive ability, and they will not play to their full potential if you play them in a midfield 2, which puts too much defensive onus on them, and restricts their attacking freedom. They play best closer to goal, drifting between the lines, playing between the midfield and the attack.
Saying Pogba is not good at defending, is not a criticism. Neither was Wesley Sneijder(although he was a diff kinda player). It doesnt matter. Not all midfielders need to be excellent at defending. And regardless, Pogba is better at defending than a lot of pure #10s, but is not as good as a deep lying CM should be because he is not one. Its not his game. You cannot put someone like Jack Grealish in a midfield 2 and then complain he is not good at defending.