Because cops are put on a pedestal. In a courtroom setting they are not treated like a regular citizen, or a party of ill-motive. Yet more and more they are. Of ill-motive, that is.
There’s a reason for that, the same reason that affirmative action creates incompetent brats who also lash out when someone tries to hold them to account.
I have had that very discussion here on FR. Look at something as simple as a traffic citation. If you go to court and say you are not guilty, then the cop says you are, what happens? You are guilty
What if you had someone riding with you and they say you did not commit the violation? You are guilty.
You are always guilty, unless you can conclusively prove that you did not commit the violation (video, proved physical presence somewhere else etc)