Uh...yeah, in large part they are - https://publicintegrity.org/inequali...of-the-police/
It's absolutely a mixture of all parties, with efforts in more recent years to address longstanding systemic issues being aggressively fought against by police unions eager to maintain the status quo and protections that they currently enjoy.
Unions aren't the single problem, though there's plenty of evidence to suggest that they may be one of the major problems in this equation in the current moment, and that historical events like the "war on drugs" heavily contributed to exacerbating many of these systemic issues and those problems persist today.