They have been trained to fear the people they're supposed to "serve and protect". They've been trained that someone not instantly acting on a command is most likely a deadly threat.
And too many police departments are filled with people who just want to play bang-bang. Actual intelligence and empathy is seemingly regarded as a bad characteristic for new recruits.
There's lots of blame to go around (the individual cops, the culture in police departments, the steroids, the training, the lack of availability of disabling tools that don't risk death, the vast numbers of guns in America, the lack of drug and mental health treatment, the lack of jobs and housing, etc.). I hope that the events over the last few years will finally get departments to change their hiring, their training, and their whole mindset about what their purpose is. It's not to be domestic warrior. It's to serve and protect. But that will take money, and we know how difficult increasing funding is...
Cheers,
Scott.