It is so difficult to put in an effective vetting system to weed out the racists, especially when you have an institutionally racist justice system from top to bottom.
Some people may think it's absurd to link it to slavery, but immediately after the civil war, when slavery was abolished with the ratification of the 13th amendment, the south still needed help rebuilding the economy after engaging in and losing a 4 year long civil war and thus losing the right enslave millions of people whose free labour the entire south was built and relied upon - they found a loophole in the 13th amendment which stated that neither slavery or involuntary servitude shall exist
except as a punishment for a crime whereof the party shall have been duly convicted - this was a reasonable condition to freedom that exist in every country to this day, but in America in the period immediately following the end of slavery, black people, former slaves, were routinely being charged with minor infractions such as loitering, wolf-whistling at white women, gathering in groups etc and then sent back to work for the former slave owners who just happened to also be sitting on prison association boards and law enforcement association boards - they were now owned by the state rather than the former slave masters.
This was the start of something that has had lasting effects still felt today. Black people were routinely painted as criminals; rapists, murderers etc, almost animal-like in every publication going. The idea that being black was being unintelligent, uncivilised and inherently violent was born, the film the birth of a nation, really hammered home that message, resulting in racist lynch mobs and vigilante justice spreading like wildfire. Over the ensuing decades black people have been mass incarcerated based on the very same stereotypical perceptions established by the former slave owners post-slavery. It's been built into the psyche by the decades and decades of propaganda against the humanity of black men. To this day, black men are viewed as inherently more dangerous than white men, hence why we see police officers calmly talk to an angry white man armed to the teeth with army style machine weapons while they feel the need to pull a gun out and shoot at the tiniest bit of hand movement by a fully compliant, unarmed black person they're talking to.
Forgive me for the lengthy post - it was in no way intended to be this long or come across as lecturing people on history. I just wanted to shed some light on how America has ended up with the society they've got today.