Why can't prosecutors convict cops?

It drives me crazy!  You see the police do something that's just nakedly illegal, it's caught on video, it's morally and legally reprehensible.  And yet there are no charges brought, or if charges are brought they're undercharged.  And even when it goes to trial the case is somehow lost.

It constantly makes me ask "why can't prosecutors convict cops?!"

I've seen so many cases of botched prosecutions of police officers over the years it's begun to resemble a pattern to me.  The police circle the wagons (and even if the brass suggest the cop was dirty and they distance themselves from the dirty cop in question, the police union holds no such concern and will defend the officer no matter what, as will the willing lackeys in the press).

My first thought is that typical prosecutors are so in hock to the police that they're simply unable to bring quality prosecutions.  When you've spent your whole career justifying whatever police do, made all your connections and promotions through being a backer of police, and have your whole worldview centered around a police way of doing things, it's hard to step back from that and begin seeing how this process clouds you in the case of prosecuting police.

Here's an example of the differences between the way prosecutors and defense lawyers try a case.  The DA has a police report and treats it as gospel.  Sure there may be mistakes on it, but generally, it's the sign of a hard working police officer writing down his general thoughts about what happened on the day in question so posterity can take a look and use it to convict some bad guy.  However, to a defense lawyer, the police report is a treasure trove in little nuggets of missed information, misstatements, errors, omissions, and contradictions that we use to tie a police officer down to a particular account of what happened.  When we can show that their report is inconsistent with other objectively provable facts, we hammer the witnesses obvious bias in how they wrote a report that was clearly false.  And through this we win trial - lots of trials.  It's really central to our way of doing business.

So imagine that a defense lawyer was tasked with prosecuting a dirty cop in a case where they wrote a report about how they were under attack when they shot someone, only to have a video surface later that contradicts that report.  From the DA's perspective, they have an inconsistent statement, but DAs are used to proceeding as if the police report is generally a true rendition of the incident that happened.  They are far less likely to use that as the lynchpin of their prosecution of a cop.  However, us defense lawyers are used to using that false police report as the basis of our undermining of the police witness.  So while prosecutors start getting caught up in things like narrative, expert witnesses, use of force experts and other police officers who will (often reluctantly) be used to bring down their fellow officers - us defense lawyers would step outside of that completely and go after police the way we're used to going after them - with their own words.

I have rarely heard of stories of the police report which understates the level of force or overstates the level of resistance being use as the lynchpin of a prosecution case - it always boils down to expert witnesses for them, and other cops being called in to question the tactics.  Why not step completely out of that role and just go after these officers in particular?  When they try to bring in their use of force experts to show that the offices were acting correctly, it's a pretty simple cross examination - "sir, if the officers were acting correctly, and they knew it, then you would expect them to write a truthful and honest police report wouldn't you?"  And then you just hammer that witness with the police report.  Don't get down into the weeds with these pro-cop use of force experts, because these guys are always saying the same thing - police are justified in any use of force any time with any amount of violence if they are in "fear."  Well, rather than counter the reasonableness, counter the "fear" by showing they lied.

There's a lot of other stuff in there, and I'll come back to more of it later.  But to me, the basis of any prosecution against dirty cops must be the lies they told on their initial police reports that they wrote prior to the video of the incidents actually arising.