I guess it stems from a time when all you really had to work with was physical evidence and the only records being what officers had written down. So it really wouldn't have worked any other way. A suspect is always going to say whatever makes them seem innocent. "I wasn't holding that knife when you arrested me" would be impossible to disprove if the suspect was wearing gloves at the time or forensic evidence wasn't yet available. So in some ways the officers word had to hold more weight.

It seems like the only solution is for every single interaction to be digitally recorded.