Ok. Using the traffic court rolling through a stop sign example again.
When I call "THE CITY OF ____" (which is the plaintiff listed in the court papers) to the stand, who will show up? The Mayor? The Chief of Police? The dog catcher?
None of them were present at the scene, what are they going to testify to? None of them personally had their life, liberty, or property trespassed on.
They may have a police officer's report, but the only thing they can do is read it. Its somebody else's words that they are repeating so its hearsay.
The police officer them-self is only a witness, not the injured party.