During review season, every manager's worst nightmare is to not have any evidence that they can label someone a low performer. Once one is identified, it's in the manager's interest to keep them in that low performer state for as long as possible. This makes it easier to reward high performers when raises and promotions are zero sum within a team. It also signals to upper management how keenly they can assess talent. In short, having an underperformer on the team is a wonderful asset for a manager.