It’s an issue in many other countries as well and there are a great many contributing factors.
Race and “Tough on Crime” politics - Ever since the emancipation of slaves on the basis of race, there have been political figures passing discriminatory policy that allows police to pursue and harass people at their own discretion: black laws, Jim Crow era laws, forced segregation, the 1994 Crime Bill, etc.
“We have to strengthen our laws when it comes to mob violence, to make sure individuals are unequivocally dissuaded from committing violence when they’re in large groups,” Florida state Rep. Juan Fernandez-Barquin, a Republican, said during a hearing for an anti-riot bill that was enacted in April.
It’s clear that you can convince people to deregulate and militarize the police if you convince those people they have a greater enemy. You can see these stances and policy directions mirrored across Europe as refugees and immigration from poorer countries have increased in the last decade.
Lack of Centralization - The FBI is in charge of investigating police departments, and sometimes you see jurisdiction overlaps which allows other agencies like the DEA, State Marshals, Sheriff’s Department, etc to investigate each other, but in general a Police Department is held to no standard but their own until things have already escalated past a point of return.
Some federal programs have tried rewarding PDs that behave well and adhere to specific training or standards, but it’s far from enforced.