A couple of reasons off the top of my head. They investigate their own crimes that they commit. The fines are paid by the public and not the officer themselves. They should be made to have insurance like Doctors. The Judicial system let's them off all the time with probation and shit when they do get charged. Last but the most important the Damn Police Unions have to much power and influence. Politicans on both sides of the aisle love Police unions, but shit on all the other unions.
Police Reform is key. Dearming the Police should also be a priority. A peace officer trained properly will never need a firearm. SWAT and special units have a need but most traffic type cops don't.
They're really not as important as you think. They only "clear" about 50% of the murders and violent crimes in the US. For crimes like theft and property damage, they basically don't even bother. They just write a report and tell you to take it up with your insurance company. Less than 15% of property crimes in the US ever see a person arrested for the crime.
Important to note that "clearing" or "closing" a crime only requires arresting someone and recommending prosecution to the District Attorney. They don't even need to make sure it's the right person or make sure the person gets convicted and sent to jail, and they STILL only clear HALF of the murders in the US.
Police sit around on their asses leeching massive amounts of taxpayer dollars, doing next to nothing. We need to take away a lot of their funding and shift it to housing programs and other social support programs, which are what actual reduce crime rates.
Because the professional police force was first established to maintain slave society in the South. Then it was extended to maintain wage slavery in the industrial centers. Then when private slavery was abolished they were retooled to re-enslave freed black people by selectively enforcing new laws targeting behaviors of freed people (prison population went from 95% white to 95% black in a decade). Then they were extended to enforce the convict leasing program where the black people that the cops arrested were leased to former slave owners to make profits for the government. Then they were extended to take orders from factory owners to break strikes up to and including murdering unarmed workers. Then they were extended when local cops wouldn't kill their neighbors, so the state police were built based on the US occupying force in The Philippines.
They have always been this way. They have never stopped being this way. They cannot be reformed. They must be fully demolished and a net new structure must be rebuilt that supercedes the few legitimate parts of their role in society.