In Golden Wind, we know that Giorno can heal people by restoring parts of bodies to people. We have seen that he can heal a body so much that Bucciarati's soul, instead of moving on to the afterlife, instead stays in his body. We saw that Giorno healed his own body and jumped right back in after Narancia's death. Now Polnareff, when we last saw him, was in the body of Coco Jumbo. Because Polnareff's soul did not go to the afterlife, he was stuck in the turtle. Why then didn't Giorno go to Polnareff's body and heal it, then stick Polnareff's soul back into the body?
It's not addressed canonically as far as I'm aware but I can think of at least two possible reasons:
It's too risky: Giorno was able to heal his own body and transfer his consciousness back because he used his own stand to do so. Either Gold Experience Requiem can't transfer a second party's soul between bodies or they aren't willing to risk the possibility that Polnareff's soul would vanish in the attempt
It's a temporary solution: As we saw with Bucciarati, Gold Experience can't completely reverse death, just temporarily bind the soul to the body as it decays. Given the choice between this and staying a turtle, Polnareff may have chosen the later.