Then they are in a position where they stand to look like fools if the truth comes out. That is why, in the Stock case, they mounted a full-court press to get the girl to incriminate the people they had already charged, even when she insisted they were not involved. They weren't interested in the truth. All they cared about was saving face - not having to admit they were wrong.
It was the same way in the Nicarico case. Even after they had Dugan's confession, and his DNA in a semen swab from the victim's body, they tried to make the case that two other guys had acted as his accomplices. Ditto the Norfolk Four - the police railroaded them all, even after Omar Ballard confessed and told them he acted alone, and DNA evidence tied him to the crime.