Iirc you are not from the US but also from a country where people are similarly violent and criminal?
Have lived in many places, from Europe to SE Asia.
The same would apply to any of them.
Most European police forces are pretty good, still many flaws, but they do a pretty good job overall.
You really think the German police are that bad that you would prefer to do without them?
You expect a shoplifter to carry a weapon and violently resisting arrest. In such a society a professional policy force is probably the better solution.
It's just a fact that some shoplifters do carry weapons. Whether the average one does is beside the point, it is an eventuality that a volunteer would have to be prepared for.
I happen to live in a society where violence and criminality is low and everyone short of a terrorist would make his situation worse by resisting arrest and knows it. I can see volunteer police working. Ymmv.
People will do many things if they think they can get away with it.
Germany is comparable to most other Western European nations in terms of crime rate. There are gangs, thugs and organised criminals same as everywhere else.
Also, unlike fire volunteers, you don't have wealthy and powerful interests actively trying to subvert the system for their own benefit. Areas of weak law enforcement also actively attract criminals. There is far more dynamic feedback into a law enforcement system.
You might be able to get a volunteer force that covers some tasks in rural Bavaria, but scaling that for Berlin, Hamburg or the Ruhr Valley with the exponential increase in complexity and level of criminality is not something that is remotely practical imo.
Scale matters.
Of course your experience leads somewhat to misanthropy.
I'd say realism, so we'll have to disagree on that
Rational, middle-class, educated professionals always tend to overestimate the representativeness of their own thought processes: "I'm a rational educated member of society willing to play by the rules for the common good, there's no reason why everyone else can't do likewise".
That's why they are so vulnerable to buying into arguments about things that should work 'in theory', and never cease to be amazed when things work out differently in practice.