This is exactly the source of the problem.
If all leftist cops quit, the police will obviously have a right bias. You should encourage people who agree with your ideology to join institutions that have power.
Same with the military. Why is there a disproportionate number of nut jobs in the military? Well, part of the reason is certainly because leftist social circles straight up bully anyone in their own ranks who would consider joining. edit: this applies e.g. In Germany much more than in the US. I crossed it out to avoid getting criticized by the us defaultism crowd.