Because Bruno plays well behind the forward and has been our best player since he signed regarding output, so changing his position just to accommodate Pogba doesn't make sense, and we don't even know Pogba will be that good playing behind the forward. I thought for years his best position was on the left side of 4-3-3 and not behind the forward ? It's like the opinion about his "best position" keeps changing as years go on.
Plus, selling Pogba might actually help us fun Sancho deal next summer. Matic, VDB, Bruno, Sancho, Martial, Rashford, this this will be overall a more balanced lineup.