> OK, but cops are also subject to civil suits
Civil liability for ones own criminal acts (at least wilfull ones, and often wilfull rather than merely negligent acts more generally) is often prohibited (the exact boundaries differ by jurisdiction), for very strong public policy reasons, and the vast majority of the acts of concern with police abuse are (very frequently unprosecuted, but that's not material when it comes to whether the civil liability is insurable) both willful and crimes, not mere negligence in either the general or professional malpractice sense (almost always intentional deprivation of rights under color of law and/or conspiracy against rights, and very often wilfull/intentional violent crimes on the assault to voluntary manslaughter to murder spectrum.)
Drawing analogies to medical malpractice misses the fact that medical malpractice covers mistakes that fall short of the professional standard of care, but don't cover when someone who happens to be a doctor just decides to murder someone.