Maybe Dallas PD were accustomed to kicking a confession out of a suspect they thought was guilty. With a confession and guilty plea there's no need for evidence. Maybe that's unfair but they certainly seemed to be quite blase about a lot of things in the case.
Fritz's testimony regarding not taking notes because they were old school and had been trained to have reliable memories is astounding considering he struggled to answer many of the WC's questions. Maybe he was better when he was younger.