Why? They are not like for like players. Camavinga is a defensive midfielder, who can also occasionally play box-to-box. Pogba is a playmaker who plays from the midfield with a decent physical presence. SMS and Pogba would literally be a like for like substitution.
Also, SMS this season has 7 goals and 10 assists from midfield from 35 matches, which is really good. (de Bruyne in comparision has 8 goals and 16 assists in 34 appearances). SMS' record is still pretty good! And he has the physical presence. Camavinga has 1 goal and 2 assists in 35 matches.
Also, by all reports, Camavinga was headed for Madrid. And he is more a DM than a playmaker.