If it's that rare, then shouldn't police be stripped of immunity completely?

Let's follow your logic. You claim that police abuse of power is exceedingly rare. In that case, to earn the public trust, and to punish the few rare cases that police do abuse their power, shouldn't the people be as free as possible to pursue justice in those cases?

If police abuse is rare, that argues for even more strict laws against police abuse, less legal protection, and more empowerment of the citizenry to address those few rare cases.