Because some white people believe it is their right, the heritage, their privilege to do so.
Because some white people inherently fear being in the same space as black bodies.
Because some white people understand the threat the police represent to black bodies and they want to leverage that threat to continue to keep their white bodies in a privileged position.
Because they are racist
As I'm sure you know, none of this is about logic, facts, or being reasonable.
If I find it I'll post it, but I read a very good article a couple years back about "White Flight 2.0".
The first white flight happened after the civil rights movement and whites fled the urban areas to segregate them selves in white neighbors in the suburbs.
The white privileged people can no longer flock to their special bubbles and be completely segregated, so now we have white flight 2.0, where the privilege white are fighting back by calling the cops of black people in their neighborhoods or parks.