Because finding people that are 100% ethical is extremely difficult. Even if we are wildly optimistic and say 20% of the population is 100% ethical. You aren't likely to weed out unethical people, so you are hiring people, training them, and then firing them 4 out of 5 of them. There are many cases where an experienced but occasionally unethical worker is better than an unexperienced but ethical worker. When faced with this dilemma it is likely that more police debts would simply cheat or cover up police abuses to retain valuable staff or staff at all.

The solution is not making humans more virtuous but reducing the capability and the harm done that unethical humans can do.

> If we cannot trust them with the very basics of ethical behavior they are absolutely in the wrong job and there need to be very clear consequences.

Police should not be trusted because they are police. There should be audits and controls that prevent abuse and unethical behavior. Small unethical behaviors should result in corrective measures but not termination, since when the punishment becomes too great you create incentives for cover ups or scapegoats. A small number of minor punishments, that catch people as soon as they step over the line, functions better as a deterrent than a large scale punishments that are unlikely to be actually enforced. Granted if a police officer does a major crime, they should face serious consequences, but the goal should be to creating a system that makes major crimes by police less likely. If they know they will get caught for minor crimes, they are less likely to commit bigger crimes.