So when the police officer says "And then he shoved me off the bridge into the stream. I don't have a recording because the camera shorted out." you find that plausible. When the citizen says "After I filed the complaint, officer Briggs just 'happened' to stop my car a block from my home... and that was the one day when there was an 'accident' with his recording device." that's fairly persuasive.
If they can prove neglect of the evidence or deliberate tampering, of course, that would be another story.
We expect the state to follow processes strictly. The penalty for failure to do so isn't personal incarceration for the prosecutors or police, but rather failure of their case. This seems like a tradeoff that works in practice.
recording everything is not a panacea. not having everything recorded a sign of nefarious actions.