It's an ad hominem attempt to deflect attention back at the attacker. Being wrong is hard to deal with, so you change the subject. It is a fairly subconscious reaction.

It's a recognition that there are complications beyond the simplistic picture one can get without personal experience. This often includes time, budget, and policy constraints the individual has no control over.

Except that there are many examples of police that do their jobs very well. I'm not sure why it has to be a mystery what standard police should be held to.

If airline pilots crashed half of their flights we would be right to criticize them. Having a hard job is no excuse for doing it poorly.

And if police catastrophically fail half their interactions, we should criticize them, too.

We only end up discussing the tiny fraction of police cases where things go badly.

What percentage of airline crashes is acceptable? What percentage of police abuses of power is acceptable? Half was a made up number. The point is we are right to hold the police to a high standard and demand solutions when they fail in their duties.

Yes. And when an airline crash occurs, we don't just say, "it's because the pilot is a horrible person", or worse, "all pilots are horrible people." We analyze what went wrong and what we can do better, then we engineer the system to accommodate human weakness and prevent damage from human error.

It's easy to be an armchair pilot, but usually crashes are caused by more complex issues.

Yes airplane crashes are complex, that's painfully obvious. In the case of an airline crash we don't say "well flying is complicated this is bound to happen" or otherwise make excuses for the pilot.

If we were as diligent with our police forces as we are with commercial airlines we wouldn't have as many bad cops.